It seems pretty much confirmed we can see Ethereum going under $400 these days.
Qn - Is it a good buy under $400 or wait for even more price drop?
Yes its good to buy ETH now. This month is time to stack some good crypto. If you have available funds then buy at top 10 crypto. Never ever put all of your position in one coin. You must diversify, the minimum is 5 coins. that is the best i think.