Run a compression check on all four cylinders. Write down the max pressures on each. Should be close to 100 PSI. The #4 cylinder may be lower. (Compression rings may be OK, but might have bad oil ring.) Whichever cylinder is noticeably lower than the others, squirt some oil in that cylinder and run test again. If pressure goes up, compression rings are bad. If pressure doesn't change, problem could be in valves. Then you would want to pull the head and check valves, seats and springs. The only other way I know to get oil in the cylinder is through a bad valve guide. Before I started tearing down the engine, I would do a little more troubleshooting. One other question--are you seeing much crankcase blowby coming out the breather when the engine is running? And make sure you have good plugs and good copper wires and all plugs are firing good.

Report back if you can and we'll go from there.