Installing Ram

AFAIK there doesn't have to be a RAM module in every slot. My workplace has a couple of 2011 iMacs and at least one of them has only 3 of the 4 RAM slots occupied and it recognizes all the RAM that's installed. So I believe you would be able to use a single 4GB module instead four 1GB modules.

I'm thinking it should be new enough to recognize 4GB module. I know the older iMac like mine can only have max of 2GB per slot but thats 2007 model.
I would almost bet your 2011 can handle max of 8GB or 16GB, so either way the 4GB module you buy should work!

All you answers can be found here: http://support.apple.com/kb/HT1423?viewlocale=en_US#4

Check out this page on everymac.com. All 2011 iMacs (Model IDs 12,1 and 12,2) can actually use up to 32GB RAM.