If the gross distribution is fully taxable and there are not any exceptions then 1040 line 4a should be blank. See the excerpt pasted from the IRS 2019 instructions:
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
A. Except
as provided next, leave line 4a blank and
enter the total distribution (from Form
1099-R, box 1) on line 4b
