It really depends on the subject and if I taught it before. But to answer your question, yes, it can take four hours even more to prepare for a 50 minute lecture for a difficult subject I have not taught before. On the other had, if I am teaching, say calculus, and I just finished teaching chapter 0, limits, now I am ready to introduce the derivative, I can do this with zero preparation time. If I am teaching the second law of thermodynamics and I want to illustrate the concept of a reversible process and use some animated gifs in a power point presentation, this could take a full day.