if you are sure it's really 4 cyls missing, i'm betting the timing chain jumped and possibly bent the valves on the driver's side head. they will jump up to 4 teeth, run like crap and NOT bend valves. could also be 4 leaky injectors, 4 bad plugs or 4 bad wires. but, 100k is about the limit for the timing chain and 122k is well past, assuming the odometer still works. could be a bargain. if the valves aren't bent, it's not a bad job. you have to pull the valve cover on both sides to be sure. good luck, chuck.