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.
5e Dungeon Masters Guide p.83 has special rules for parties with less than 3 players and more than 5 players. The official 5e rulebook clearly states that parties of 3-5 use the standart rules, while 6 players and more need balancing adjusment. "The preceding guidlines assume that you have a party consisting of three to five adventurers ... If the party contains six or more characters..."
3.5e Dungeon Masters Guide p.11 "The normal-sized group is around four players (with the DM as the fifth person)."
Party size of 4-6 is established as "normal" in 4e DMG p.31, but then it considers party of 6 being a larger party: "It’s too difficult to listen to six people who are all trying to tell you what they do at the same time."