Spending BPS on 4* is a waste since you get like 1 4* every 2 days from factory. And all of them eventually will be replaced by 5*.
BPs are only for 5*.
There are reasons to get 4th slot as factory is basically like a f2p gacha using common materials and can give you high tier units.
Still it's all up to rng.
Saving cores for events is a good idea, I've completely forgot about that.