Toyota Repair: P0304, engine light on. 98 tacoma, 4 cylinder., unnecessary work, compression test

Question
My 98 Tacoma P/U 4 cylinder, manual Trans. 139,000 miles. Idles rough, runs rough and backfires some. According to the code the 4th cylinder is misfiring. I changed the plugs. I don't want to do a lot of unnecessary work on it. What else should I check. If it's a complicated procedure for an amateur let me know that too.
Thanks,
Doug

Answer
If the misfire is there all the time check the engine compression, these engines have a problem at times with the valves getting tight or leaking, they are adjustable with shims, I would recommend a compression test, if the misfire seems to come and go and is not consistent, one of the fuel injectors may be the problem, this is also common, if the compression is good then replace the #4 fuel injector.