Step 1  Solve the Centers
This is how to solve the centers on a 4x4.
The Steps

Solve the First CenterThe first center can usually be solved in only about 5 moves. In general, you want to build a pair of two center pieces, build another pair of the same color, and then match them together. Sometimes you may be able to take shortcuts so that they both pair simultaneously or build the first pair so that the second pair will match easier, but really, this step is pretty simple and very intuitive. Once this center is solved, put it on the D face. 

Solve the Second CenterNext, solve the opposite center. The strategy here is pretty simple. With the first center on the bottom, first make a pair of two center pieces in the E layers and insert it into the top layer using Rw U2 Rw'. Next, make another pair of two center pieces in the E layer and insert it the same way. Be sure to insert it with the solved pieces in the correct position so that you don't lose what you've already solved. Sometimes you will notice that pieces that belong in the opposite center are already placed. If this happens, look for shortcuts for solving the second center. Put the two solved centers on the L and R faces before continuing. 

Solve the Third CenterTo solve the third center, we have a lot of freedom. From this point on, most of your moves should Rw, Lw, and U turns. Make a pair of two center pieces and put them lined up vertically on the left. Then make a second pair of two centers and match it up to the other pair to complete the summer. Once you've done this, rotate the cube to put this center on the B face. 

Solve the Fourth CenterThe fourth center is solved much in the same manner as the third center except there is a little less freedom because of the already completed center. Just be careful not to do anything that will affect the completed center on the B face. Once you've solved this center, rotate the cube to put it on the B face. 

Solve the Last Two CentersThere are only seven distinct cases for this step (not including the solved case) and they're all very intuitive, but I've included algs for these cases below. 
Last Two Centers Algorithms
hese cases are all pretty intuitive, but I have included the algs here just in case you're stuck on any particular case or just want to confirm that you're doing it efficiently. For each of these cases, the U center should be blue and the F center should be red.
Diagram  Algorithm  Comments 
Rw U2 Rw2' F2 Rw  The two centers are solved but need to be swapped with each other.  
Rw U2 Rw'  This is one of the nicest cases and easiest to recognize.  
Rw U Rw'  This is probably the easiest case to execute. 

Rw U' Rw2' F2 Rw  This is sort of a mix between swapping two centers and the case above. 

Rw U' Rw' U Rw U2 Rw' 
Reduce into pairs of bars, AUF, and finish the centers.  
Rw' F Rw F' Rw U2 Rw' 
This is the same idea as the alg above. The first four moves are just a sledgehammer.  
Rw U Rw2' F Rw 
This one is pretty easy and a little similar to swapping two centers. 