What are the names of the grain you are feeding and what is it's protein, fat and fiber %?
To me, feeding a growing horse only 1 cup twice per day is no where near enough, and you should do it by weight (lbs) not volume (cups). I have fed all my weanlings and yearlings (depending on their body condition, which your little 4 month old looks a little pot bellied so he needs more protein in his diet and be checked for worms) about 1-2 lbs minimum per day. My yearling at the moment is rather, well shall we just come out and say it....."fat" (cresty neck and hard to feel ribs and hip bones) so he is getting 1 lb per day of a 10% feed split into two daily feedings, plus all the hay he can eat and limited pasture turnout.
Your yearling looks to be of good weight so just make sure she is getting enough to maintain that plus allow for growth. As she grows, adjust her feed to her appropriate age/weight and make sure she has nice flesh over the topline, hips and ribs to keep her looking good.
Both are pretty little ones and have fun with them! Don't forget it's never too early to get them working on their leading skills and manners!