upgrade my imac

if i would like to upgrade my old imac memory slot, and there are two slots already been installed with two 4g memory.

would i still be able to uninstall these two and changed the four slots into 8gb of each and end up with 32gb in total.

if not, would i be better off install 8gb to each of the rest two slots, which ends up with 24gb in total. Or keep the same size and install 4gb of each one and end up with 16gb in total.

Thank you!