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. Currently Ethereum is at $440 and I believe it is going to go under $400 with the stats the coin is currently holding.
Also, I think Ethereum under $400 will be a good buy as though the prices could drop more, it is better not to take chance then.
Ethereum with its successful projects and wide acceptance in the financial market, is a great coin to make investment into. The coin's idea of smart and ledger contracts which can hold state and arbitrary code is quite revolutionary and increases the coin's potential.