Ahh. The source of confusion is the idea that SVR4 is the basis for anything :-)
COFF is the base SVR4 format; a.out is the base BSD format, and is based on older (Bell Labs) unix formats. (I've yet to see a unix system that didn't use the *name* a.out for the default compiled executable, but that's got nothing to do with the contents...) So: SVR4 can't run a.out; it is a BSD format (which means that it's the SunOS 3 and 4 format; SunOS 5 (ie. the OS part of Solaris 2) is SVR4 based, and supports ELF by default, and a.out only as a way of being backwards compatible with sunos4.
