The choice of “four” in particular has no special relevance. The idea is more that it’s some arbitrary, “random” selection off of a fast-food restaurant’s value menu (which are numbered, and typically ordered by number—this is faster, more efficient, and less likely to be misheard. It’s also less personal, less “human,” and more arbitrary and mechanistic).
There are a few potential considerations for the number used:
Those menus vary in size, but most of them have a fairly limited number of options, so only a small-ish number would fit.
#1, #2, or #3 might have connotations associated with sporting competitions (gold, silver, bronze) that are too positive for the statement the author is making.
Besides, choosing something too near the beginning weakens the sense that the choice is arbitrary: the whole point is how impersonal and arbitrary the fast-food system is. There’s no real reason #1 couldn’t have been just as “arbitrary” as #4, but humans are bad at “random” and tend to read too much into things. So ideally, the number used would be as “un-special” as possible to hammer home the point that we are dealing with something arbitrary.
So 4 may have just been chosen as the first number that didn’t have those problems.
More likely, though, the author did not think very hard about the number they chose, though. Most likely, they just picked the first number that came to mind—which may have come to mind because the author is human and has similar tendencies to read connotation and pattern into things that don’t necessarily have them, and so subconsciously avoided #1, #2, or #3.
Alternatively, it’s plausible that the author was referring to an actual value meal #4 that actually was the most popular meal in America, or some American town, or whatever. Say, for example, that McDonald’s value meal menu at the time had three types of hamburger as #1, #2, and #3, and then #4 was a Chicken McNugget meal—and for whatever reason, their McNuggets were more popular than their Big Macs—then maybe #4 actually was the most popular meal at the time. McDonald’s and other fast-food restaurants change up these menus from time to time, and popularity changes too, and besides the author may well have had popularity in a particular town or anecdotal popularity based on those they personally knew, so short of asking the author, it’s unlikely we’d ever be able to confirm that.
But again, I doubt that’s it. I am pretty convinced it was just an attempt by the human brain to choose a “random” small-ish number.