I'm not suprised that you're finding it hard to get hold of Maya 4.0, it's pretty ancient. What kind of plugin is the one you need? Is it one of those Maya pseudo-dlls or a MEL script? If it's a MEL script then you can look right at the plugin source and probably fix it up for a later Maya version; if not, you may have a problem. Have you tried the plugin with a more current Maya version? Maya PLE is free for non-commercial use; theres no harm in downloading it just to check if the plugin works or not...
Overall, I'd suggest migrating to an new engine... Working with a now-unsupported engine is guaranteed to be a source of pitfalls..