Trying to benchmark, computer restarts, Ubuntu 14.04

[epixoip]

I have 4x XFX R7990, and i have a 1600W Power Supply. Do you think that would be enough?

Not even close to enough power! So this is almost certainly your problem.

The 7990 actually will draw just over 450W when doing things like brute force attack against MD4 (which is the first benchmark ran). So you'll need a minimum of 1900W for the GPUs alone, not including everything else in your system. If I were you, I'd pick up a second 1600W PSU.

You may also need a new motherboard as well. Inspect the ATX connector for signs of damage. This is what happened when we tried to run 4x 6990 on 2000W of power: