The previous owner told me he had the timing belt replaced, so I checked it out and the timing was spot on. However, he did say the belt was "making noise" before that though, so who knows what happened. The other valves were tight, so I should be able to use one and see if it seals in #4.
I didn't see any nicks on the piston, but it does have some carbon on it. I'll clean it tomorrow and get a good photo. Thanks everyone for helping.