How can you expect an answer when nobody has had the opportunity to play SHIV yet?
Anything could happen. SHIV could suck, in which case we'll go back to SHIII. Or SHIV could just be a Pacific remake of SHIII with some bits and pieces added, in which case we'll probably play both or maybe migrate to the new one, or... SHIV could be significantly improved in key
gameplay issues, making it impossible to go back to SHIII even if you tried.