you do not have an underpayment penalty. an underpayment penalty is when you do not pay in enough taxes by the due date of the return. this is completely different from a late payment penalty which applies to any payment due which is paid after the due date of the return. you have a late payment penalty because the amount due was supposed to be paid 7/15/2020. the late payment penalty is 1/2% a month. as an option don't pay the $4 just the $500 and see if the IRS bills you.
