My 5 year old has done that, and I've provided childcare for school age boys and they do that too---when I'd pick them up they were famished by the time they got to my house, where I already had pre-prepped most snacks. I think it's just a normal growing child thing. Sometimes they just need snacks! I remember eating snacks after school when I was a kid, too.
The most successful snacks for starving boys, in my opinion: whole wheat bread toasted and then spread peanut butter on it and serve with milk (or cheese toast: put some cheese on the bread and toast it in the oven until the cheese is melted and bread is toasty). Also, sliced apple wedges with a little bit of peanut butter and some apple juice. Or even some nachos sometimes: spread the beans and top with a little cheese before going to get him, and it'll just take a few minutes to heat that up.
As Czechma mentioned, the snacks need protein. Crackers are not going to cut the hunger unless you slice a little cheese or something with it. And fruit is good, but again, maybe a pear with a cheese stick on the side, or apple with peanut butter. A little bowl of macaroni and cheese. Or one slice of bread with a little ham and cheese, heated up and folded over for a "half sandwich". Those kinds of things. (Another thing, I'm sure this is just mental but sometimes warm "feels" more filling).
Young boys are burning energy just sitting around growing, and especially if they're active and playing a lot, you don't need to worry about "oh, a nacho or peanut butter aren't low fat". It's twinkies and ho hos that can be a problem, but for a normal active child, things like cheese toast and milk won't be a problem at all. I also agree that you can do a more filling snack and move dinner to slightly later (we eat about 6:30 except on kung fu nights). We do not eat dinner at 4---I agree with you there. We like to eat dinner together as a family, and unless the moms that are suggesting serving dinner at 4 serve their children completely different "kid meals" then "adult" dinner isn't ready that early and dad is not home from work. My 2yr old and 5yr old kids don't eat nuggets, fish sticks, and little burgers at a separate time, they eat arroz con pollo, gumbo, grilled salmon, and thinly sliced steaks with asparagus---with the family.