To get the $400 price all segments need to be in the same booking class. If one segment is in the higher booking class the higher fare will apply. If its a round trip fare you can have one fare basis on the outbound and one fare basis on the return.

The only time you may have mixed in one direction is if you are stringing together 2 diferent fares. Say they are selling a T fare LAX-BOS and a V fare BOS-LHR you can combine them. In this case you will be charged whatever the base fare is for LAX-BOS + the base fare BOS-LHR + any taxes and fees