If bitcoin will hit $6000 then it will probably end up with $5800 or $5900 ethereum can hit $400 but that does not matter there will be bounce for me.
Normally cryptocurrencies price should not be correlated to each other but most of the time, it just follows the bitcoin price, and I agree with you if BTC will drop under $6000, ethereum price will drop under $400 unfortunately.
As everyone knows most of the crypto's are correlated to the Bitcoin price, so Ethereum hitting below $400 can be of no surprise once Bitcoin slips below $6000K.
But from my point of view Ethereum would hold its $380 level, if $400 is broken, these are just my personal views.