As with SPARC, there is an emulation mode that should run the majority of well-behaved SVR3 (iBCS) binaries. Most SVR3 stuff appears to work under Solaris 2.4.
Applications from any other vendor's standards-conforming 386/486 SVR4 should also run.
However, some vendors have made incompatible changes to their SVR4 release and programs linked on those versions may not work. Future versions of Solaris 2.x for Intel will address some/most of those incompatibilities. Unixware is one of the offenders.