If we talk about systems, rulesets, etc. then for DnD campaigns a party of 4 is actually the gold standard.
No it isn't. Even officially, it is party size of 4 to 6. The pro-4 side is just conveniently interpreting that as exactly 4.
In my over 25 years of playing D&D, in TT, play by email, and nowadays on Roll20 using 5e, I have never ever played a game with a party of 4. The smallest parties have been 5, and typical parties have been 6 or 7.