I agree that 4 hours is low but the problem is that nowadays many kids just can't sit and learn for that long.
there's a limit to the productivity you're going to get when you just force kids to learn the entire day. For many people, that time would be better spend doing something else even if in the perfect world they would gain more from learning during that time.
If kids can't learn for longer, the method of teaching should be reexamined and reassessed. I recall when I was in Yeshiva I was asked to learn Gemara with one of the staff's children. For the first few sessions, it was real torture, the kid kept on looking at the watch, and couldn't wait for the time to be over, and was clearly disinterested. I then managed to figure out a way to bring across the Gemara in an animated and engaging method. The kid didn't look at the watch even one time, and learned more in one session than he did during all other sessions combined.
And if one cannot figure out how to teach in a way that the kids will want to sit and be engaged untill the very last moment, then turn to
On a separate note, and somewhat tying into language skills, I proposed to a few Menahalim that a curriculum of Jewish Stories (preferably in Hebrew/Yiddish, but obviously it depends on the school) be incorporated into the schedule. The main benefit of the stories is to increase אהבת השם, אהבת התורה and יראת שמים, but a side benefit would be increased vocabulary and language skills.
You can never have too much or regret time spent on סיפורי חז"ל, סיפורי צדיקים, סיפורי חסידים, etc.
