I started playing during revised, specifically with the Fallen Empires expansion, and thought that the 4 card limit had always been a part of the game. I looked at my Pocket Players Guide from around that time and could not find that rule. After a little bit of research I ran into this comment on reddit, https://www.reddit.com/r/magicTCG/comments/2jxr9z/comment/clh3ip7 which seems to have an answer although their source linked no longer works. In 1996 the mirage rule book has the 4 card limit listed under "house rules" and in 1999 the Sixth Edition Comprehensive Rules Document has the 4 card limit listed as rule 100.2

Again unfortunately the link to that document is broken so we can't be 100% sure, but it seems the answer is sometime between Mirage in 1996 and Sixth Edition in 1999

Four card limits were implemented in the 1990's; I remember it when Ice Age was released.

It doesn't apply to just basic lands, yeah. But any special lands you can only have 4. It's been a rule for as long as I've played (since 2017), but I do know it wasn't a rule at the start of the game. I think they added it pretty early on though, as a response to people making decks out of just channel and fireball for instant wins.

And, sure, you could keep the ratio of card types the same, but while I don't play Yu-Gi-Oh, I have to imagine there are some cards better than other cards. So to make a deck that big, you'd have to include cards that just aren't as good. Playable, sure, but I can't imagine it finding its best cards consistently enough to be competitive.

It was a rule in the early 2000s when I started playing.

I started playing magic around 2004 or 2005 and I believe it was in the rules then. And no, basic land cards don't abide by the 4 copy rule, but any non basic land does