Mainly I see two reasons to buy AOS4:
1st: if you are developer you can test your OS4 versions (that is if you maintain or plan to maintain an OS4 version of your software
2nd: Curiosity.
I must confess I am curious how it performs on the Peg, but when I think about it I can hold my horses and get an impression at some user meeting.

As for sw AOS has, MorphOS hasn't: AudioEvolution is said to be nice. But if you have PSA already, it doesn't offer too much extra. Other OS4 soft, dunno, surf OS4 depot what's there.

And don't expect OS4 to execute more legacy sw than MorphOS does.

I'd say it also depends on your wallet's size.
If it is filled plentily: go and order.
If it is reasonable filled: get more info and/or check it out at some meeting and consider an order.
If it is tight: forget it.