I have only two sides that are not right but the other two are right... everything but that has done. How do I complete this 4x4x4 Rubik's cube?
This is an adjacent corner PLL parity case which can happen on even-layered cubes (like this 4x4x4, 6x6x6, 8x8x8, etc.)
Put green at the top and the two incorrect corners at the top-front (as in the picture above), and use the following algorithm (Here is an animation for this algorithm):
(Uu)2 (Ll)2 U2 l2 U2 (Ll)2 (Uu)2 F' U' F U F R' F2 U F U F' U' F R
where a capital letter is the outer later, and a lowercase letter is the inner layer (i.e.
Uu is the top two layers;
l is the inner left slide; etc.)
Why do parities happen on a 4x4x4 which aren't possible on a 3x3x3? I (Quuador) and some others explain that here in a recent question on the Cubers-reddit:
What exactly causes 4x4 parities?