Using all four DIMM slots demands more of the motherboard than using just 2. Some RAM is OK in two slots, but not in 4. (See the QVL for your motherboard.)
The DIMM pairs aren't identical, so the board may not operate them together if you're relying on automatic settings. One possible solution would be to set the memory timing and voltage manually in the BIOS setup. Use the slower timings of either pair (9-9-9-24), and set the voltage to the rated value (1.65V). It may still fail, but it'll probably work.
I believe that you'd have to use something other than the default settings in the BIOS to get 1600MHz anyway. (It would default to 1333 or even 1066 MHz.)
Britton30's Memtest suggestion is a good one.