I have php4 working with pws 4 and windows 98 but it took a while....I actually had to reinstall pws after i had the settings correct for it to kick in...but hopefully you won't have to do the same.
first I had to alter php.ini
I changed doc_root = path/to/inetpub/wwwroot
I did this because i send and test all my php files from there.
I assume you edited your reg file to point to your php4isapi.dll file then double clicked it to register.
I also had to alter hkey_classes_root in regedit.
with hkey_classes_root selected, i clicked edit->new->key and gave it the value .php
I clicked my new .php key and changed the default setting to phpfile (dbl click default and change it while .php is selected)
select hkey_classes_root again, click edit-> new -> key and give it the value phpfile.
select phpfile and edit -> new -> key and name it shell.
select shell and edit -> new -> key and name it open.
select open and edit -> new -> key and name it command.
select command and dbl click default and type in the path to php.exe (make sure it's something like c:\php\php.exe)
close regedit.
Go into pws and look at the advanced settings.
Select home and click edit properties. Make sure execute is selected. close it and reboot your computer.
Hope this helps
Gary