My question is: Why all these 4 users? Will there be any trouble if instead of 4 I only use one windows user/db login for every step?
If you use 1 user, you typically end up giving that user more access than it needs. By having the example use 4 different users, it helps demonstrate the good practice of using dedicated accounts for particular purposes. If one is compromised, there’s less impact on the overall system. If the example used just one user, everyone would assume it had to be that way.