I've looked at my httpd.conf file, and saw that the installation of PHP5 made an include in this file. So in the beginning of the file, PHP4 gets loaded, and in the end PHP5 gets loaded by this include.
That's why my server admin didn't show PHP5. It showed PHP4, but the server was running PHP5.
So I've deleted the include, and changed the lines like it should. Now PHP4 is not loaded anymore, and my server admin works now like it should.
Strange thing: my server works now for 4 days without any problem. So perhaps this is the solution? Let's hope so. I'll let you know!