The difference between 4 and 5 years isn't really that big under normal circumstances. But these are normal circumstances for several reasons. This is a new contract, not a contract extension. And the cap may not go up next year, which is huge.
Under normal circumstances you work out an extension before the final year of the players rookie contract. Since he's being paid very little in his final year of the rookie deal, that gives the team a chance to dump a lot of signing bonus on that last year. That last year no longer exists.
Now it's pretty simple math. Say 4 years 35 Mil per. No matter how you slice it, it's going to get ugly at some point. Normally they start off low. His first year would be around 20M for example. But you still have 120 million left that you have to spread out over the next 3 years. It frees up cap room early and then you hope the cap goes up to keep up with the contracts rising hit. As I mentioned before, that might not happen.