Guys, rabbit trails!

He has low compression on 4 cylinder and misfires on those cylinders. That is the problem, end of story.

While it is unlikely that his rings are bad without burning any oil, its not impossible. There are 3 rings on each piston. 2 compression rings and 1 oil ring. I bet they all have specific purposes...

I had a much longer response typed into my phone, but it didn't post...

Short story, I have seen the leakage causing misfire at idle before, both times were valves, but they only missed at idle and acted fine off idle.
At idle everything moves slower and there is more time for the compression (air/fuel mix) to leak out, once the engine is spinning faster there is more air and fuel in there (the throttle is open) and with everything moving faster the time it can leak out is smaller so the leakage is a smaller percentage.

How the previous owner managed to kill the compression rings on one side, that's crazy...