There could be a few issues. It seems like some overwatering could be going on. If you grow autos in their final pot size (I do) then it it's best to center water smaller amounts as the seedling cannot use up enough of the soil moisture and will stay soaked. This can mess up their ability to draw nutes and can literally drown the roots. It also looks like a little light burn could have happened, although I would expect all of them to be affected not just plant 4. Be careful about water droplets sitting on the seedlings after watering while your lights are left on, as it can act as a magnifier and cause light damage in rare cases.
Good luck with the grow! They can all still turn out great :)