I am pretty sure that it can be compiled for OS4 with the latest SDK. I have done that many times, and I am using that version for my emails.
Well I was just a quick test, I simply replace os3 target name with the hostname in my meta buildscsript, but ofcourse the hostname is AmigaOS and the target name is os4 , nothing like consistency :-) . Just typing gmake, it builds until it fall over an ACCESS_READ instead of a SHARED_LOCK.