the darker color could just be that track 4's color is different than the others. Check to see if the track has been muted. If so, unmute it. Check to see if track 4 is being sent to the same output or bus as track 3 (or any other audio track that plays back properly).
Do any of your tracks have Solo enabled? The darker color would indicate that it is inactive, so if it is not muted that would indicate that it is turned off by something else.