So this comes up constantly. "Amiga should move to x86 and everything would be way better".
I don't think its all that simple really. Here's some things that'd at least warrant exploring:
1: Time and money required to port the operating system. I have absolutely zero idea how long this would take, but I am assuming it'd be a pretty significant endeavour.
Considering it's been ported to PPC 'just recently' (in Amiga terms), the amount of work required for x86 should be managable.
2: Time and money required to port applications.Without app's, an OS is worthless.
No need porting apps, a compatibility layer (which can probably be extracted from UAE) will do the trick.
3: Oh hey there AROS! What functionality does os4 give that AROS doesn't ?
Good question.

4: Most overlooked:
Hardware support.
AROS has been around for years and still supports a fairly limited range of hardware.
When it comes to hardware there's two options: The linux way (support it all in the kernel, which requires massive amounts of work from a large number of people) or the windows way (have the hardware manufacturer write drivers for you).
OS4 would have neither.
The driver problem exists regardless of the CPU platform.