Plants look good but I have some yellow/brown leaves Help?!?

Ok my plants seem to be doing fairly well for only 4 weeks. However I noticed a set of yellow/brown leaves at the bottom of one of my two seedlings. Could it be too much nutes? I have cut my watering back to every 3 to 4 days. However I have begun introducing nutes in the water. I didn't gradually introduce it. I followed the directions of the shultz 10-15-10 plant food. It says mix 7 drops to a quart of water. So thats what I have been watering with. Maybe I should cut back? However the plants seem to be doing quite well except those leaves I mentioned. See closeup pic. Sorry the pic is not good my camera stinks, but I am refferring to the very bottom and smallest set of leaves at the base of the plant. Also should I keep my light schedule the same for now? I have been giving them 18 light 6 dark. I think it's a long time before I go 12/12? I also want to wait a while before transplant as they are soooooo small I'm scared to do it. Your thoughts?

mine always die n fall off

Not sure what you mean by cotyledan? I just know they are the bottom set.

looks like that's all it is, you're good. Plant looks good man.. those are just it's "first" leaves.. and they are called cotyledan leaves... they do that at 3-4 weeks, as long as it's not the first set of leaves that are pot looking kind of leaves... you're good.

Thanks so do you think it's ok to keep watering with the nutes full strength? It's Shultz plant food 10-15-10 I am using seven drops to a quart of water. Seems pretty diluted to me. How about light is 18/6 cool to stay on? Or perhpas more light hours? When would you all transplant? Some say now, others say wait till week seven. I don't want the plant to get root bound and slow growth. However the plants seem fairly small and I know I can expect slow growth with four 23 watt (100 watt equivalant) CFL's!