Think you already know how to count? You'll learn some new techniques in Part 4. This branch of mathematics is called combinatorics. The counting tricks we learn in Part 4 will be extremely useful in the probability examples we run into in Part 5. So learning the topics of Part 4 will pay off twice. Often when students struggle with probability their issue really boils down to not understanding the combinatorics techniques that are require to determine probabilities.
With the simplex method, the technique is pretty much the same with all standard problems and with all non standard problems. Once you get a problem set up correctly, there's no guesswork regarding what to do. But there's a good bit of calculation done in carrying out the simplex method. It's exactly the opposite with Part 4 and Part 5. In these sections, the hard part is figuring out what technique is applicable to a particular situation. Once you've figure that out, there's generally not a lot of calculation required.