No as Creative Suite 4 is tied to the platform you purchased. You can qualify for the upgrade to CS6 and request a Mac version. You can find more details at http://www.adobe.com/products/creativesuite.html?promoid=JOLIS.
But that means buying a license - I have already paid a fee when I bought CS4 - is there not a way to migrate my license to a MAC version? I can't really afford to upgrade to CS6 yet.
Essentially I don't want to spend any money - because I have done already, and I want to carry on using CS4 for now until I can afford to upgrade.
No. Platform switching is only possible ever for the current version, meaning you need to upgrade. Read the gory details here:
If you can't afford the full deal, you can always go Creative Cloud for a few months. The current discount is still valid untile end of August, making this even more affordable and I'm reaonably sure spending those 30 bucks is in the budget even for many hobbyists....