Solve it like a 3x3

Approach
Using outside layer turns (U,D,F,B,R and L) follow the steps needed to solve the 3x3 Rubik's Cube. The outside layer turns will not scramble the solved Center pieces.

Important: There may be two times during the 3x3 solve that additional steps are needed to solve the 4x4. These two cases are referred to as parities and can only be found in the 4x4 Solution. The parity fixes are in steps 4 and 7 of this guide.

Step 1
Solve the WHITE Cross.

Step 2
Solve the WHITE Corners.

Step 3
Solve the Middle Layer, treating the two inside layers as one. This picture shows the Cube with the WHITE face down.

Step 4
Make the YELLOW Cross.

This is where the first parity may ccur when following the 3x3 Solution Guide. Determine how many YELLOW Edge pairs are on the UP face and follow the corresponding steps.

If there are 1 or 3 Edge pairs on the UP face, then hold the Cube as shown and follow this sequence to fix the parity:

If there are 0, 2, or 4 Edge pairs on the UP face, then continue with the steps outlined in the 3x3 Solution Guide.

Step 5
Make the UP face all YELLOW.

Step 6
Position the YELLOW Corners correctly.

Step 7
Position the YELLOW Edges correctly. This is where the second parity may occur. Determine how many of the YELLOW Edge pairs are in the correct position (refer to color layout in Stage 1 if needed), and follow the corresponding steps.

If there are no YELLOW Edge pairs in the correct position, follow the sequence below either once or twice until at least one Edge pair is positioned correctly. If there is one YELLOW Edge pair positioned correctly,  hold the Cube so it is in the BACK and follow the sequence below either once or twice until the the Cube is solved.

If there are two YELLOW Edge pairs positioned correctly, then the cube is in a parity state. To fix the parity and solve the Cube, determine which two Edge pairs need to be swapped and follow the corresponding sequence.

If the Edge pairs that need to be swapped are adjacent,        hold the Cube so that they are on the FRONT and RIGHT faces. Then follow   this sequence to solve the 4x4.

If the Edge pairs that need to be swapped are opposite each other, hold the Cube so they are on the FRONT and BACK     faces, then follow this sequence to solve the 4x4.

Congratulations
If your Cube looks like this, you have solved the Rubik's 4x4 Cube!