Would you suggest transfering them even now at 4 weeks old and about 12" high?

Something else that is confusing me abit is that the three other plants under the same conditions and watering schedule seem to be pretty healthy... One has slightly yellowing leaves near the bottom however that seems to be getting better since i changed the watering schedule. Is it possible that i could of over sprayed that one plant or something like that has made the leaves go hard? Also the new leaves coming through on the bad plant seem ok...?