Just saw this thread and would like to add that I've upgraded from the 3 to 4 to 7B-ST's. Each adds more weight to the bass and clarifies dense muscial passages. So, as always, your choice depends on what you like and what music you play. I like solid bass so thought it very worthwhile to move through the line. From everything I've heard, the 7B-ST's are superior to two bridged 4B-ST's. I also read a very recent review (I believe in Stereophile)that suggested the SST's don't necessarily beat out the ST's on all fronts. In fact, the reviewer actually preferred the 7B-ST to the equivalent SST for serious listening.
If you're happy where you are, stay where you are and take that first to getting off the merry go round ;-) As you can probably tell, I'm an audioholic who is still having too much fun to quit. Whatever you decide, try before you buy, preferably in your home. Happy Listening.