If the amount in box 1 and box 2a of the Form 1099-R you entered are the same then on the Form 1040 box 4a will be blank.
IRS Form 1040 Instructions page 26 - https://www.irs.gov/pub/irs-pdf/i1040gi.pdf#page=26
Except as provided next, leave line 4a blank and enter the total distribution (from Form 1099-R, box 1) on line 4b
