Four limbs are an inherited starting plan, not a mammal invention
The basic tetrapod limb plan is shared across limbed vertebrates, including mammals, birds, and sea mammals, and it begins with a limb bud formed through Sonic hedgehog (Shh) signaling [1]. That shared origin means mammals did not evolve four limbs from scratch; they started with a body plan that already had four limb buds. The review of human upper-limb development describes the same sequence—limb bud formation, axis development, segmentation, cartilage formation, digit rays, and digit elongation—as a common developmental pattern [1].
This matters for the question because it shifts the burden of proof. If mammals had more than four limbs, we would expect to see a developmental route for making additional limb buds. The supplied evidence describes a conserved program for building the limbs that are there, not a mechanism for adding new limb pairs [1].
Mammals change limb shape by tuning genes, not by adding limbs
When mammals do evolve dramatically different limbs, the evidence points to changes in when and where conserved genes are expressed rather than changes in limb count. In bats, which are the only mammals capable of self-powered flight, the wing is supported by four elongated digits within a broad membrane, and the foot has a long calcar; these are modifications of the existing forelimb and hindlimb, not extra limbs [5]. The study found that genes such as Meis2, Mab21l2, and Tbx3 are expressed in different patterns between bat and mouse limbs, and it supports the hypothesis that modulating the spatiotemporal expression of multiple conserved genes controls limb morphology and drives diversification of mammalian limbs [5].
In other words, the same four-limb framework can be stretched, shortened, or reshaped. A bat wing is still a forelimb; a whale flipper is still a forelimb. The supplied evidence does not show any mammal evolving a fifth limb, and the bat work illustrates how existing limbs become specialized through gene expression changes [5].
Regeneration is limited, but that is not why mammals have four limbs
It is tempting to connect limb number to regeneration, but the supplied evidence separates the two. Mammals can regenerate only distal digit tips, whereas amphibians such as the axolotl can regenerate entire limbs throughout adulthood [2]. The review frames this contrast as a way to understand why developmental programs fail to be fully engaged in mammalian limb injuries, not as an explanation for limb count [2].
That distinction is useful: a mammal's inability to regrow a whole limb does not mean it could not develop an extra one. The regeneration review highlights constraints on reactivating developmental programs in adults [2], while the limb-development review describes how limbs form in the embryo [1]. Neither paper supplies evidence that regeneration limits explain the four-limb condition.
The supplied evidence does not directly answer why four and not more
None of the supplied papers directly tests why mammals typically have four limbs rather than more. The human upper-limb review explicitly notes that it does not address that specific question [1], and the regeneration review likewise does not directly address the evolutionary or developmental reasons for four limbs [2]. The limb-length study examines regulatory changes in limb-specific enhancers but does not explore the four-limbed condition [3], and the humeral diaphysis study analyzes bone structure across mammals without addressing limb number [4].
What the evidence does support is a conservative conclusion: mammals inherit a shared tetrapod limb-development program [1], and their limb diversity arises mainly from modifying the expression of conserved genes within that program [5]. The supplied papers do not show a developmental mechanism for adding limbs, nor do they test whether such a mechanism could exist. So the best-supported answer is that four limbs reflect a deeply conserved inherited plan, while the question of why that plan cannot be expanded remains outside what these papers establish.
Sources used in this answer
A Literature Review on the Development of Upper Limbs in Humans
Tetrapod upper limbs share an evolutionary origin and a common developmental pattern beginning with limb bud formation via Sonic hedgehog signaling, with later steps modified for specialized functions.
Limb regeneration from amphibians to mammals.
Mammalian regenerative responses are largely restricted to distal digit tips, whereas amphibians such as the axolotl can regenerate entire limbs, revealing constraints that limit engagement of regenerative programs in mammals.
Regulatory divergence modifies limb length between mammals
Regulatory changes in limb-specific enhancers influence limb length in mammals through developmental mechanisms.
Humeral diaphysis structure across mammals
Across 164 species of extant and extinct therian mammals, global compactness generally increases along the humeral diaphysis, with modifications in specialized aerial, aquatic, and subterranean clades.
Differential Expression of Meis2, Mab21l2 and Tbx3 during Limb Development Associated with Diversification of Limb Morphology in Mammals
Differences in the spatiotemporal expression patterns of conserved genes such as Meis2, Mab21l2, and Tbx3 are associated with diversification of limb morphology in mammals, including bat wing and calcar development.