From Apple’s web site:

Final Cut Pro 4 and earlier versions are not supported on Mac OS X 10.5 Leopard.

While the current version of FC is 6, many videographers stick with older versions for the basic reason of "If it ain’t broke, don’t fix it".

If you upgraded to OS X 10.5 and wanted to continue using version 4, you’re out of luck and will have to upgrade or buy new, which will run you up to $1,300.

If you want to know if your current older Mac meets system requirements for Final Cut Studio 2, the product page for FCS2 will give you the scoop.