If it is not using oil and there are no combustion gases in the coolant I suggest doing a valve adjustment. That just might get the compression back on that one cylinder. If it doesn't help, then try some STP or Marvel Mystery Oil, maybe a stuck ring or a leaky valve. A leak-down test will tell you what the problem is with that one cylinder. You might just get by without having to remove the head. It will never run as smooth as it should with that one cylinder so low.