What's the difference between mod_php4, mod_php, and php?

What's the difference between mod_php4, mod_php, and php packages?
As directed on page 30 of 'Beginning PHP4' I ran $rpm -q mod_php4 to which I
was told 'no such package installed. I do, however, have php-4.1.2-7
installed.
Do I still need mod_php4? I found it at rpmfind.net. In fact I found all three
above 'variations' of php-- php, mod_php and mod_php4.
I wish I knew what each was used for to understand what I still need to
install.