1040-SR line 4

[EKD]

Why is line 4a blank when 4b is filled in?

[macuser_22]

There will only be an entry on 4a of the distribution is partly non-taxable or entirely non-taxable  per the IRS 1040 instructions that say:

You should receive a Form 1099-R
showing the total amount of any distribution
from your IRA before income tax
or other deductions were withheld. This
amount should be shown in box 1 of
Form 1099-R. Unless otherwise noted in
the line 4a and 4b instructions, an IRA
includes a traditional IRA, Roth IRA,
simplified employee pension (SEP)
IRA, and a savings incentive match plan
for employees (SIMPLE) IRA. Except
as provided next, leave line 4a blank and
enter the total distribution (from Form
1099-R, box 1) on line 4b.