Rubik's Cube 4x4
The 4x4 requires different moves to solve than the original 3x3, and it's more challenging! Rubik's 4X4 will drive you crazy. You may think you will be able to solve this puzzle quickly, but then you make a turn on the cube and it changes all over again. The constant changing Rubik's 4X4 will keep you entertained for hours. I was being as gentle as possible, but if you learn how to solve the cube, then you'll be performing thousands of turns on it. Both the 3x3 and the 5x5 cubes seem to hold up a lot better than the 4x4. If you're going to make serious attempts at solving your 4x4, I recommend lubricating it with something. The cube-solving sites seem to recommend silicone spray (whatever that is). When I was in high school I'd take them apart and put vaseline in them, which smooths over the "catches" that happen as you turn rows that are slightly misaligned, and also has a slightly glue-y effect that holds it together better.
Rubik's 4x4Rubik's Revenge Cube 4 X 4
Rubik's 4x4
$17.05
Rubik's Revenge Cube 4 X 4
$16.99