Make sure you have a beefy PSU for you rig as my rig pulls 620w from the wall with just 2 cards; idles at 100.
Keep in mind that because of constant power draw that mining requires that a good quality lower wattage power supply (e.g. 850W) is much better than a so so one that's much high (e.g. 1,000W).
For 4 7970 I think a good quality 1100W+ power supply is required.