Well i'm not a specialist but i'll give you my opinion.
1: Can you bring the 2x4gbs back to the shop and get your money back or exchange for 1x8gbs?
-> if yes: You should get the point. exchange it for 1x8gbs and buy the exect same 1x8gbs= 16gbs
-> if no: Just get the exect same 2x4gbs
with the theorie: if 4x8 (for video editing peeps) works and gives performence why would 4x4 not work.
Note: make sure you have enough clearence for you CPU Cooler if its a big one
Keep in mind, this is what i WOULD do.
I only use 2x4gbs and have only builded one rig myself, i'm not a specialist i just wanted to give you my honest opinion.