Nutrient question....please help!!

Could someone please post a pic of what a seedling looks like when it is just about ready to be given its first nutrients. Is there a good rule of thumb to go by. I have heard the four true leaves, but i'm not exactly sure what thats supposed to look like either. For instance seeds planted in a DWC system given only RO water for the first seedling stage, what would a plant need to look like to start adding nutes? I'm very confused on this. please help!!

You can start feeding them a week 400ppm when there are 3 full sets of leaves and the 4th are starting to show. After that bump it up a small amount with each water fill.

thanks