Mine did for some reason.
--disable-all seems to disable support for sessions, and pear doesn't seem to get included either. I think both required for Squirrelmail.
Maybe I took a wrong turn somewhere.
I did a pretty much stock toaster install other than going with Apache 1.x, and the latest squirrelmail & vpopmail (stable) versions.
Have a look at lang/php4-extensions.
Matt