In most cases, to ensure backward compatibility, servers running PHP4 are configured to also process files with ".php3" extensions. However, if that is not the case (e.g. your browser is either displaying the source code or not recognising the file format), please ask your service provider to remedy the situation.
The SPIP team can only recommend that you try to use a host server that runs PHP4 or even PHP5, especially if you wish to benefit from the most recently released version of SPIP.
Karl recommends this for those running their own servers: If you have access to httpd.conf and have compiled PHP4, modify your httpd.conf as follows: