It may be time to give a low dose of some nutrients, yellowing is usualy a sign of nitrogen deficiency. Its also the time to start adjusting the PH of your water and or nute mix going in and testing the PH of the water run off. A good routing is feed, water, water. Feed the first day then on your next watering day just PH`d water and the same for the third watering day then repeat. The link below has alot of good info on learning what your plants are telling you when you see something wrong on the leaves. Some nutes can drop the PH a full 1 point, to low and the plant isnt absorbing all the nutes it needs and to high can lead to salt build up that can lock out nutes. Thats why adjusting the PH of the water or nute mix going in is important. Also to help reduce the chance of salts building up in the soil water till you have about 20% of the water or nute mix as run off in a catch tray, also some people flush thier plants when they see signs of a problem or to just flush the salts that build up in the soil every couple of weeks. The two small single bladed leaves and the two small round ones will normaly die off but your shouldnt be this early and at that size. I would rinse the soil off of the leaves but dont leave water sitting on the leaves just gently tap the excess water off, plants also breath through their leaves. Diagnosing Marijuana Cannabis Plant Abuse Problems Charts and Photos