I started four San Marzano tomato plants from seed a little over a month ago. I repotted them a few weeks ago into 4-inch pots. The plants were doing fine until a few days ago. All four of them are having the same exact problem. The cotyledon leaves began turning yellow and then drooped and died. Now, the next leaf up has turned yellow, and the leaf above that is losing its dark green color. If you look closely at the last picture, you can see the transition from green to yellow. There is still some green near the tip of the leaf. I'm worried that the plants are going to die. These were potted using Miracle-Gro potting mix, so I have not used fertilizer (because they claim to have slow-release fertilizer in their soil mix). I have been afraid to use fertilizer because I don't want to "burn" the roots. I water them every couple of days when the top inch of the soil is dry. All of the leaves on all four plants were completely green until a few days ago, and now they're all turning yellow from the bottom up. Should I add fertilizer the next time I water? Please help.
Well, your plants seems dandy fine to me.
It looks like your plants are advancing and growing, naturally discarding of the oldest/lowest leaves.
This does not look like any sort of tomato disease i'm aware of. It might be early stage root rot, which is unlikely, because it's a 1 month old plant in a new compost mix. It could also be nutrient deficiency, but that it also unlikely because of the potting mix. Here's a link for a very comprehensive booklet about the subject (it's in hebrew, but all disease names are in english so you can google them)
If i were you i'd experiment a little bit - if you have 4 of the same plant, you can try and do some a/b testing on them... just try adding in some mild fertilizer to one of them. If it was a nutrient deficiency, thae plant should react in a matter of days (for most nutrients). other variables you can play with is temperature, placement / sunlight, size of pot, and so on