MarlinG was talking about seedlings not getting enough light, getting leggy. That is they stretch up to try to find light. The stems are more slender and there is a lot of stem length between the leaves. It is not a good thing. My tomatoes got ahead of me this year and I tried moving them in front of a window. They got very tall and leggy and then in the hardening off process with a lot of moving in and out etc, a number of them just snapped, because the stems weren't strong enough.
But this would NOT be what you are talking about if your plants are no more than 4 inches tall. Are they spindly and tend to just flop over on the soil? Then what happens, do they keep on growing?
You could be getting damping off, it's a fungal condition of seedlings growing with too much moisture and humidity and not enough air circulation.