why do i lose lp if i get fourth place? wtf

What a questionable name you have there.

it happened to me too once.

but it was like 3 lp so whatever

This happens when the relative MMR of the other players in the match is low (when compared with yours) and is more likely to happen at higher tiers when the pools available to make a good match is limited.

I have seen this happen for even higher placings to master/challenger level players when streaming. The same applies, but in terms of getting more LP, when you are leveling up initially, which is how you can get huge LP for a win early (say you are in Bronze 2 and playing Gold/Plat but still come first).

(Note: I have no inside knowledge here, just observations and historic knowledge of how MMR and matchmaking works)

Stop playing with your friends and screwing up your MMR.