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?
even if the price of Ethereum drop off 15%, its much better to wait and buy it on its lowest price. Its not profitable to buy it just because the price went down $400, its still high. Think about it. When the price falls down unto its lowest and buy it, the next thing will happened is its price will bounce higher then time for you to sell it earn an extra profit on from it.