Seedling Problems

My plant is currently on day 6 of vegging. The first set of leaves are starting to come about. Everybody knows when you first start a seedling that it eventually has 4 leaves come out in a couple of days. The problem that i am having is that the two main leaves are lush green but the other two leaves are turning yellow, stale, and hard but the other two are fine. I am wondering what is the problem??? And what i can do to help my plant grow all lush green leaves???? Also, how to make the plant recover????
 
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