If I buy 2 16GB RAM cards to fill the other 2 slots, will these work along with the two 4GB that come stock for a total of 40GB RAM? Or do they all need to be the same size in each slot?
Yes, they will. That would be my plan. Order stock ram (2x4GB) from Apple, get 2x16GB and install myself for 40GB total. I've been using 16GB at home in a PC, and 20 and 32 in 2 different iMacs at work with no problem, so 40 would be a dream!