For a computer doing nothing but mining, I usually assume ~100W for the system. The best way to measure this is when the system is at idle, not mining, with 1 GPU installed. Most of our systems are usually around 75W, but I assume 100 just to be safe. This gives you some forgiveness when the HDD spins up, CPU actually does something, etc.
Our 5x 280x rig was pulling 1450W while mining. I undervolted them, and now they're drawing 1250W.
That means my kill-a-watt is telling me that my cards were running at ~270W/each, and now they're running at ~230W/each.
Whoever said 1500 didn't do any math. That's way overkill for 4 GPUs. As I said, I wasn't even pulling 1500W on my 5 GPU rig, at stock volts.
I think 1200W is your best bet. This will let you run all 4 at stock volts, or give you some headroom if you do undervolt.
I would say 1100W is on the low end, but is doable if you plan on undervolting, or if you just don't care about headroom and want to push your PSU to the max.
You MIGHT be able to squeak by with a 1000W, IF you undervolt the snot out of them, use a low power system, and don't mind pushing your PSU >100% load. I would no recommend this.
I would say this is your best bet:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151109Another option that might be cheaper is two PSUs. Two of these would power all 4 GPUs + your rig + powered risers, and only cost you <$200, rather than >$300.
