I'm a math major and as far as games go I only can think of one that I played for months. It is not a video game, though I still highly recommend it. It's called 24.
4 people play, and there is a deck of cards in the middle. Each card would give you 4 numbers and you have to be the first one to figure out an arithmetic combo to get to 24. For example a card would give you:
2, 6, 10, 11
one answer could be
11 + 10 = 21 + (6/2) = 24
I think this is your best bet for 4th grade.
http://www.amazon.com/Original-Game-Cards-Single-Digits/dp/B002AODZFQ