The main draw for me is sample library quality and sound access.
In the MAX version there's 8000 sounds. I am told the meta data is grouped by mood type and instrument.
If a search brings up what I need in seconds, this means as much to me as the sample quality.With 8000 sounds you NEED this. I can already duplicate anything in something else, but I can't get to it fast.
If it has both sound quality and easy access, I see ST4 becoming a strong contender. ST3 quality won't make the grade for serious composers. They claim they've upped the ante. I am optimistic.