The first step to solve a 4x4 is to solve the center pieces as the 4x4 does not
have center pieces.
Next, solve the edges by matching the seperated edge pieces.
Finally, solve the 4x4 like a 3x3
There is a weird thing about 4x4, and that is parity errors. These is due
to the absence of center pieces in the 4x4. There are two parity cases
and there are algorithms to solve each of them.
Now you have a completed 4x4!
Below is an in-depth tutorial on the 4x4 Beginner Method
For online tutorials, click here!
This method can be applied to bigger cubes such as 5x5. See video below