Mix two different types of 2 GB RAM in iMac
As a general rule, you can mix sticks of RAM as far as brands/speeds/timings/sizes.  

Your computer should accommodate different speeds/timings by scaling down speeds/timings on the faster sticks until they match the slower sticks.

Brands will matter as far as they do for everything else: everyone wants you to buy their brand of stuff, but they'll generally work. I've stuck random brands into random Apple/Windows machines and the ones that aren't just terrible do always work.

Depending how far apart the specs on your sticks are, you may have stability issues, but those are easy to fix - remove the stick you added.