Is is often thought that line 4a is the total distribution***, and line 4b is the taxable amount. It can happen that line 4a be smaller than line 4b because of the convoluted IRS instructions for line 4 on the 1040.
***Line 4 is for reporting IRA distributions. Line 4a is often thought to be the total distributions while line 4b is the taxable distributions. But because of the fine details in the 1040 Instructions for line 4, sometimes you do not add the gross distribution to line 4a, but you always add the taxable amount to line 4b.
The result is that sometimes line 4a is less than 4b, sometimes 4a is equal to 4b, and sometimes 4a is greater than 4b. This is not you and it's not TurboTax - it's the IRS's instructions.
Note that the IRS made a number of changes to form 1040 and to this line in particular from 2017 to 2019, so what was true for one year is likely not true for another year.
This response is valid for tax year 2019.
[Edited 3/10/2020 1:52 pm CDT]
