Amazon has a target compensation for you for your first 4 years, with a cash signing bonus making up for the lack of stock in the first two years, so it evens out.

Of course the 5/15/40/40 model only applies to your signing; all further yearly stock grants are over the next 2 years from grant time.

Is it common to get additional grants during that first 4 years?

It's common to get a second vesting in your third year (or else your comp would go down). It's almost common to get a promotion around that time -- managers are responsible for promoting their employees, and it's taken seriously.

It also means your destiny is in your manager's hands, which can be unfair if you end up with a bad manager, or several new ones. And unfortunately at Amazon, this is very common (current employee here).

That's literally every job ever. You aren't going to have a successful career at Google if your manager doesn't want you to. The org golden handcuffs are gone, so if you have a bad manager, change teams.

For the record "bad managers" are no more common or less common at Amazon as any other company.