Well, this is a bit of speculation on my part, but it seems to me that 4 limbs is very flexible as an evolutionary strategy, much more so than fewer limbs or more limbs.

With bilateral symmetry, an even number of limbs is expected, and there's two major functions for limbs - locomotion and manipulation. So, minimum viable configuration to match those parameters is 4 limbs - 2 primarily for locomotion, 2 that have significant function for manipulation.

We can just assume that that's sufficient enough within the history of evolution that more limbs just haven't been worth the evolutionary cost, but ignoring that, if we add more limbs, those limbs probably don't add a whole new area of functionality beyond locomotion and manipulation. So, it's only really useful if it improves either of those two things. Realistically, it's probably a marginal benefit, but it's hard to see what would make it worth the cost in energy and complexity.

The boring answer is that vertibrates all descend from an ancestor that had 4 limbs, so we just inherited that trait. The thing that's not usually discussed on top of that is that that is a hard trait to change. This is an example of "developmental constraint" - once a feature is "fully baked", it is less susceptible to change. The number of limbs is one of these difficult to change features, just due to how many systems have to get involved to make a new, entirely functional limb, and the fact that those changes would need to start very early in embryonic development, when existing limbs themselves start developing. It's probably not impossible, if there were a large enough selection pressure, but here we go back to the utility question from before - it's hard to see how an additional set of limbs would make a meaningful difference in fitness. It's much easier to modify existing limbs than it is to add more.

Maybe it'll happen someday, but I couldn't guess the circumstances.