She can earn up to 4 credits per year. That means that your wife can earn one credit for each 1360$ of wages (as in 2019) or self-employment income.
Do you want to know more about the credits per year?
Yes, please.
So, as I said, is 1 credit for every 1360$ she earns so she'd get her fourth credit when she has earned 5440$ and that would complete her 4 credits per year.