Unfortunately, that is almost sure the problem. The back cylinders burn valves because of the exhaust crossover heat. I you tear it down to fix it, you definitely want to remove both heads and have both heads checked and the valves ground. It is a big job, I don't think I would recommend most non-mechanics do it. I just finished mine in November and it's a pain, and I am a full time mechanic.
It also is fairly expensive, if you have the machine shop adjust the valves, you are looking at about $600 just for the machine shop. If you adjust the valves yourself you can save a couple hundred, but will probably have to spend a hundred for valve shims.
If you try to do it yourself, you will probably get very frustrated halfway through, it is not a fun job. Hate to be the bearer of bad news, but that's the way it is. I'm not recommending any particular course of action, just telling you my experience.