well, when you installed php, it should have placed the module somewhere in the apache directory. Search for the module:
find /usr/local/apache2 -name libphp4.so
if you can't find it, search the PHP directory as well:
find /usr/local/php (assuming that's the path) -name libphp4.so
once you find it, place it in the directory that apache is searching...
mv /path/to/libphp4.so /usr/local/apache2/modules/
Now give it a try and cross your fingers.
PS - you can also specify the module path by adding this line to the configuration command
--activate-module=/usr/local/apache2/modules/libphp4.so