Most often 5 CAS Latency rated RAM requires less voltage to run than 4 CAS latency. I'm not familiar with your MB, but if the BIOS allows you to adjust above 2.2 to 2.3 volts to your DRAM frequency, you can run the higher voltage required CAS 4 modules. Basically the lower or 'tighter' the timings, the faster response time your DIMMS have with your CPU. By increasing the voltage many high performance DIMMS like Corsair rated at 5 CAS LAT can achieve 4 CAS latency timings.
Gigabyte will not support you. If (and almost certainly when) the thing won't boot with flakey implimentation they will simply fob you off - no investigation, no support, no bios update just a "get stuffed" response.
Seriously, every day of my life I have to drop into the "Setup" menu on a Gigabyte Motherboard, let it warm up while makeing a cup of coffee and then boot on exactly the same settings that run day after day never a failure.
Gigabyte support really really is non-existant and I am not the only one with these sorts of issues with their boards.