This link should tell you everything you need to know: http://www.sram.com/sites/default/fi...ranksets_3.pdf

The 4 hole component is a self-extractor cap. There should be a hex recess bolt visible in the center of it. when you back that bolt out the extractor cap will push the crank arm off the splined axle (a very handy feature).

If you truly have a 1/2" of play at the crank arm, something is seriously wrong. You may have had a bolt back out and if that happened, chances are your crank is toast as the splined interface will be permanently damaged. Read through the PDF and follow the instructions for tightening the crankset. Without seeing the bike firsthand it's difficult to say how severe the problem is. Hopefully a re-torque fixes it.