I think the problem is rather that what you're asking isn't a simple thing to accomplish.
One of our clients needed to have MySQL 4 for an older PHP app that doesn't support MySQL 5. After I spent about 8 hours digging through alternative options (like downgrading MySQL, replacing her application and rewriting all the SQL calls within the application), it became obvious to me that those options were not really feasible.
Instead of downgrading (and making all the rest of my users upset), we just installed a second instance of MySQL just for her. After the installation, within the "MySQL Instance Config" tool, we named the new instance "MySQL4" and set the port to something other than the default (which was obviously already taken by MySQL5).
Within her application we had to change the mysql connect line from simply "localhost" to "localhost

ortno". This went absolutely flawlessly.
If this is the situation you're in, and I suspect it is, then I'd recommend you just install MySQL4 as well.