If we are taking about Plesk 7.5.2 for WINDOWS!!! then there is a good chance you can do this.. Please test first.
Last weekenD we had a disasterous upgrade to our Plesk 7.1 system. Sorting throught the mess allowed me to discover the following.
Plesk 7.5.2 runs 2 copies of MySQL. One for itself and the other for the user databases.
These are totally seperate and reside in different locations on the HD. The Plesk one is at port 8306 and the users one is at port 3306
Now - MySQL say you CAN run different copies of MySQL on a system so long as you have different ports, so it is probable that you can install a new version (Say MySQL v5) for your users and let Plesk do its own things with it MySQL 4.1.7 install.
There are some issues I can see. You might not beable to let your clients create their own ODBC connections or manage their own databases via Plesk (We use a tool SQLYog to do that from a remote machine).
But if your machine is a fairly closed machine that you manage yourselves with a small number of clients then it makes it a lot easier.
Just my views.. Please test first
Dave