So i just purchased a used R530 and it is currently confirgured with 4x 4GB modules for a total of 16GB. Is it possible to add a 8GB module and if it is what would be the correct configuration.

Generally not a good idea to mix RAM.  Would not work if one is ECC Reg and other is LRDIMM for instance.

Might just look at buying a 4 sticks of matching 8GB RAM.  Check out cloudninjas website.  Got some DDR4 upgrade deals.