Your only required to mark down 1 quarter in each half for each player. So if you get to the 4th quarter and 1 kid has already played his 2 quarters that were marked and another has as well that normally plays the whole game and you wanted to play the other kid who doesn't play more than 2 quarters typically then you could play him in the 4th, but as far as pullings kids in and out of an ongoing quarter no it's not allowed.