PHP4 from layman overlay doesn't build mod_php for apache2. Reproducible: Always I found out that it uses the no longer existing env var ${APACHE_VERSION}, so it skips the whole apache2 part. Adding APACHE_VERSION=2 at the beginning of the ebuild works as a quick fix.
Thanks, I committed the suggested change, but didn't test it. Please keep in mind that php-4 is *really* old and is subject to multiple (probably severe) security problems. You're on your own regarding that.