I am afraid that IF you want to play with PHP versions, install/unisntall etc. YOU must be[come] familiar with UNIX/PHP/SQL.
If you have root access and YOU definitely do not want PHP3 support, you must recompile PHP4 withOUT --enable-versioning in PHP4 Makefile.
If you have root access and want [temporary] to disable PHP3 support, play with Apache's apache.conf configuration file (comment any line concerning PHP3 modules. (assuming PHP4 is compiled as DSO)
If you don't have root access: I don't know, don't ask me :-))
In general - RTFM, i.e. read PHP documentation included in the tarball. It's completely enough !!!
Hope this help.