The 4x4x4 was originally marketed as a revenge on people who had learnt to solve the 3x3x3, many people try to solve it layer by layer but this is a terrible idea, the basic beginner method is to solve the centres, followed by the edges and then finish it off like a 3x3x3 (unless you get parity...), modern 4x4x4s generally perform very well but the mechanism is quite complicated. The best speedcubers can solve the 4x4x4 in 20-25 seconds on average.