Been struggling with this issue for a while but have managed to make it work. And it actually is not that hard. To do so:

*presumes one is using c:\phpX for folder names where X is version number.

1) decompress php4 and php5 to separate folders.
2) Create a subdirectory in those folders called "run"
3) For php4, place php4ts.dll, php4ts.lib, php4embed.lib, php.exe and your php4.ini file in said folder.
3) For php5, place php5ts.dll, php5ts.lib, php5embed.lib, php-cgi.exe and your php5.ini file in said folder.
4) Open IIS admin MMC.
5) Under home directory, add script mappings:
a) PHP4: extension .php (or whatever); path: c:\php4\run\php.exe -c c:\php4\run\php4.ini %s
b) PHP5: extension .php5 (or whatever is not for A); path: c:\php5\run\php-cgi.exe -c c:\php5\run\php5.ini %s
6) restart IIS.

If php4 is working already, you really only need worry about the php5 part of the above instructions. This setup will allow you to use php4 and 5 with different configurations and extensions, etc.

Sorry for berevity, typing one-handed here.