I have a 2001 Nissan Sentra. My service engine soon light came on and I took it to a parts store where they will plug in their little diagnostic computer and it read - Misfire on cylinder 4. So my brother changed the spark plugs and the light stayed on for a day or two and then went out. It came back on, and I went back to the same parts store and they used their diagnostice tester and it gave the same reading as before - misfire on cylinder 4. What do you think it could be this time?