4b / 7b - Historically the ST series had the option for 2 ways to bridge it, the SST series was selected at build, I dont know about the SST2..........
In LOOSE terms without going into detail the 7B is BASED around a 4B.
There are reasons why you would buy a 4 or a 7, the 4 does give greater scope, the 7 is dedicated monoblock, these will both suit different uses and that is system specific.