Look for terms like 80 plus, bronze, silver, gold, and/or platinum rated.
The higher the rating (platinum > gold > silver > bronze), the more efficient power wise it is. But the more it'll cost you up front.
You'll need a lot of dongles to run 4 cards on one power supply, or have pricey power supply.
Having two power supplies is actually pretty simple. You short out two pins on the main power connector to turn it "on", and the rest is the same. It will cost you more in the long run with electricity that way, but if you have cheap or free electricity, it might work better for you. If your electricity is expensive, you probably want one power supply.
I have a 4x7970 and a 4x5870 setup, each with two power supplies. Not the most efficient in the world, but this way I have half the 7970s and half the 5870s on one 15A circuit, and the other half on another 15A circuit. That wouldn't be possible with just two power supplies.