With tomatoes, over-fertilizing generally just makes extra leaves and less fruit - it takes an awful heavy dose of fertilizer to make them sick. If nothing is eating the roots, then yellowing leaves sound more like a red flag for disease, especially if your other plants are doing well. Fusarium wilt is a common culprit. I would dig up the sick plant, discard it -not compost it- and be careful not to plant tomato family plants in that spot.

Also, do you smoke? If so you may be carrying the spoors of Tobacco mosaic on your hands and given it to the tomato plant.
Linda C

sometimes you buy same plant at the same place at the same time and some doesn't make it. it's not uncommon. don't worry about it too much, just pull it out and plant another one and or enjoy the 4 that you've got. it's hard to any one to really tell what's wrong in somebody's garden without seeing it. my guess is that this 1 plant is just a bad seed then there isn't much you can do or did do. if another one follows and the next, that there's some practice that could be corrected. so think first about the most common factor. are you watering too much? are they getting enough sunlight? these two thing are the most common factor that kills the plant before any disease and pest. don't water until the first 2 inches of soil gets dried and tomatoes needs full sun 6-8 hours. so please look in to these to before you consider anything else.