So I just had a look in Interest payments. Many things are not clear about this.
Is 4.00% an annual rate?
Since the founds are not used for any purposes that would generate profit why are you doing this? Out of the goodness of your heart just to make your users enable 2fA?
The longer you have some funds on site the more often you will earn interest.
Above that you menioned that the interest is paid every 4 hours. How does that make sense?