Have I caused engine damage by cranking it backwards?

As long as your timing belt was still attached you should be okay. When turning an engine in reverse, the only things it can damage are the oil pump and possibly the water pump. You'd have to do it QUITE a bit though, four revolutions shouldn't hurt it. The reason it CAN damage the engine is because you're forcing the internal mechanisms to run in the opposite direction they were engineered to turn. Like I said above though, you'd need to do it quite a bit. Many vehicles require you to actually rotate the engine backwards to install transmissions and valve-train components; Honda, Toyota, and Hyundai are perfect examples. I've worked on quite a few Subarus and I've had my timing and crank shaft slip backwards. I never had an issue.

The term "Interference" just means that when your camshaft and crankshaft timing are not cohesive, the piston will hit valves once it reaches it's TDC. As long as you had your timing belt on BEFORE you did this, you should should be fine. If not, rotate the engine VERY slowly with a breaker bar or long handled ratchet and wait to see if it catches anywhere. I worked on a Hyundai Tiburon and the person before me was off by a couple of teeth on the timing belt. It started, but sounded like a tin can full of marbles (The valves slapping the pistons!). I hope this helps you!