4
Xoops 2.x uses several functions, variables, etc. which are only available since PHP 4.1+. Most are available in 4.0.6+. With PHP 3.x you will definitely run into problems with missing functions, probably missing globals, and likely incorrect function calls (different numbers of arguments etc.). You might also run into trouble with the MySQL version, depending on which version you have available to you.
I was not around during the development of XOOPS 1.x but I believe you will run into the same problems there.
