Four legs good, six legs ARE better. The most successful land animals have six legs organized into two tripodal superlegs moved one tripod at a time, so you get perfect stability in mid-step. That cannot be said about the four-legged creatures, especially the first ones. Furthermore, there is something disturbing about the precept that the four legs had helped the ancestral, semiaquatic tetrapod to transition to land. Just look at the [fully] aquatic mammals. Do you see four legs? You see a couple of strong flippers and (at best) the vestigial hind legs. The latter are of no use because the thrust comes from the vertically undulating tail. The hind legs get in the way. The reptiles and amphibians beat their tails horizontally, but the hind appendages are retained only when part of the life cycle is spent on land.
One would expect that the transition to land followed this pattern: the pectoral fins of the tetrapod-like lobe-finned fish used for locomotion, with underdeveloped hind limbs, the mode of land locomotion being crawling (similar to mudslippers) - then, over time, hind legs developed and walking became the way. The fossil record suggests otherwise. The hind limbs were more developed than the front limbs even in the fully aquatic ancestor. There was no crawling on land, it was walking from the start. The creature was using its four limbs in water, where having hind limbs did not pay off in all of the subsequent fully aquatic tetrapods. The ones that clung to their hind legs, like basilosaurus, used those for docking during copulation or other acrobatics.
For years we have been told just-so-stories that our four limbs were a land adaptation. In the past few years this story has crumbled apart, and it is no longer even considered. It is not supported by the fossil record and developmental genetics. The story of tetrapod evolution, the real one - as opposed to the one imagined to conform to the textbook notions - looks more mysterious than ever.
...[It was] assumed that the evolution of limbs was initiated and driven by the colonization of land. Here, however, was an early tetrapod that was ill-suited for life on land. It had well-defined digits (fingers and toes), but no wrists or ankles. It had relatively long limb bones, but they couldn't support much weight. Its hip also couldn't support much weight since it was weakly attached to the spine. Its short and thin ribs were incapable of protecting vital organs. Acanthostega also had a deep tail which sported a large bony fin. In short, it had a tail suited for swimming, a fish's spine and paddle-like limbs. A primarily, if not exclusively, aquatic lifestyle for Acanthostega is further indicated by the presence of internal, fish-like gills. Acanthostega's small, fish-like nares (nostrils) were probably used only for smelling under water; air may have been brought to the lungs by gulping. With its combination of fish-like and tetrapod features Acanthostega has engendered a variety of speculation about the paleoecology and evolution of early tetrapods. Its feet may have been superior to fins in negotiating shallow waters filled with aquatic plants and woody debris. http://www.devoniantimes.org/Order/re-a
...the first tetrapods arose from advanced tetrapodomorph stock (the elpistostegalids) in the Late Devonian, probably in Euramerica. However, truly terrestrial forms did not emerge until much later, in geographically far-flung regions, in the Lower Carboniferous. The complete transition occurred over about 25 million years; definitive emergences onto land took place during the most recent 5 million years. The sequence of character acquisition during the transition can be seen as a five-step process involving: (1) higher osteichthyan (tetrapodomorph) diversification in the Middle Devonian (beginning about 380 mya), (2) the emergence of "prototetrapods" (e.g., Elginerpeton) in the Frasnian stage (about 372 mya), (3) the appearance of aquatic tetrapods (e.g., Acanthostega) sometime in the early to mid-Famennian (about 360 mya), (4) the appearance of "eutetrapods" (e.g., Tulerpeton) at the very end of the Devonian period (about 358 mya), and (5) the first truly terrestrial tetrapods (e.g., Pederpes) in the Lower Carboniferous (340 mya). Physiol Biochem Zool. 77 (2004) 700.
The lobe-finned fish like Eusthenopteron is said to use its "limbs" to walk on lake beds, though there is no evidence for such mode of locomotion (extant lobe-finned fish do not walk on their fins). Furthermore, it had six fins. Another tetrapodomorph fish, Panderichthys, lost its dorsal fins retaining only paired pectoral and pelvic fins. So our tetrapody originated in the loss of unpaired fins in the ancestral fish that elongated its paired lobe-fins, for a purpose unknown. This purpose probably had no more to do with walking than basilosaurus' hind limbs. It could've been sexual selection. It could've been supplying sudden thrust for ambushing prey. Spreading weeds has been suggested, too, although I do not see what hind limbs would be for. It could've been anything. In all probability, we will never know why tetrapod-like fish started to grow their lobe-fins, however, no matter what that reason might be, they would have exactly four limbs. Why?
Because this follows from the developments that occurred 135 Myr before the first tetrapod-like fish appeared. That the land vertebrates would have four limbs with digits had been decided 500 Mya. The stories about the creativity of evolution should not be taken at the face value; many decisions are irreversible. A vertebrate animal may be in desperate need of six limbs for its survival. It is not going to get those extra limbs. To understand why one has to realize that the first, jawless fish did not have paired appendages that became our limbs. Their fins were on the midline. Then two sets of paired appendages appeared in jawed fish. The current thinking is that these paired fins were not new. The skeletal patterning has similarity to that of gill rays. See http://www.pnas.org/content/106/14/5
There are three other theories (external gill, lateral fins fold and fin-spine ostracoderm theories). The gill arch theory is that the first pectoral and pelvic girdles were behind the gill arches from which these evolved. The external gill theory is the same idea but the gill is external, as in lungfish. The lateral fins fold theory suggests that paired fins evolved from median unpaired fins. The gill arch theory is the current favorite, but it has been in and out many times. The lateral fold theory postulates a duplication event involving Tbx genes patterning the fins (an extreme beneficient mutation, not unlike polydactyly). That was an extremely rare event, perhaps itself part of the whole genome duplication event leading from the jawless to jawed fishes.
Evolving paired fins improved the control of movement by tail beating. The two sets came from linking the patterning genes to the ones guiding anterior-posterior patterning. That we will have exactly four limbs has been decided the moment this genetic toolbox has been settled upon. In the subsequent 500 Myr no variation of this toolbox has been made. The potentiality for the complex anatomy was there right from the beginning. The tetrapods were not innovative at the fundamental level.
...the genetic and developmental toolkit that builds limbs with fingers and toes was around long before the acquisition of limbs, as this toolkit exists in a living primitive bony fish, the paddlefish. The latter have an elaborate fin skeletal pattern similar to that seen in more primitive vertebrates such as sharks and many fossil fish. Accepted theory among scientists has been that the pattern of Hox gene expression seen in zebrafish represents the primitive condition for the fin in any vertebrate, and the group leading to tetrapods elaborated on this Hox expression by adding a second phase and added to the skeletal pattern. Tetrapods have a second phase of Hox gene expression that happens later in development. During this second phase, hands and feet develop. Although this second phase is not known in zebrafish, it is present in paddlefish, which reveals that a pattern of gene activity long thought to be unique to vertebrates with hands and feet is in fact much more primitive. This is supportive of the theory that the genes to help make fingers and toes have been around for a long time. Prior to this find, a popular theory was that it was a novel development. Here's fish that doesn't have an autopod but is still using those genes in a second phase to help pattern out a fin that doesn't have fingers, never did, and is very far removed from tetrapods. It took a set of environmental triggers to make use of the pre-existing capability. http://www.sciencedaily.com/releases/20
...Genetic processes were not simple in early aquatic vertebrates only to become more complex as the animals adapted to terrestrial living. They were complex from the outset. Some major innovations, like digits at the end of limbs, have been achieved by prolonging the activity of a genetic program that existed in a common ancestor of sharks and bony fishes. The same genes that produced ancient fins enlarged their role about 365 Mya in amphibians struggling to adapt to swamps, creating a distinct burst of development and more versatile appendages. The sharks and many other types of fish do not form more dramatic appendages during the late phase of Hox gene expression because it occurs briefly and only in a narrow band of cells, compared with the more extended time frame and larger anatomical area needed to prefigure the hand and foot in limbed animals. Finding the second phase in sharks, which have skeletons consisting not of bone but of cartilage, means the genetic processes necessary to muster fingers and toes existed more than 500 Mya. http://www.sciencedaily.com/releases/20
As is the case with five digits, the four limbs have been settled upon eons before the first digits or limbs appeared. One sees this again and again. The adaptational paradigm of evolution, once one pays closer attention to the facts, poorly bodes with the reality. It is not the creative force. The benefit kicks in very late in the game when the ground work has been laid. Evolution is doing this ground work rather than accruing accidental benefits that are the fruit of its hard labors.
Why do we have four limbs?
PS. "Gaining ground: The origin and Evolution of Tetrapods" JA Clark (2008)
"Fins into limbs" BK Hall (2007)