TurboTax is handling the 4a and 4b entries correctly according to the IRS instructions. Here is what the IRS instructions say about entries on line 4a and 4b:
Fully Taxable Pensions and Annuities
Your payments are fully taxable if (a) you didn't contribute to the cost (see Cost, later) of your pension or annuity, or (b) you got your entire cost back tax free before 2018. But see Insurance Premiums for Retired Public Safety Officers, later. If your pension or annuity is fully taxable, enter the total pension or annuity payments (from Form(s) 1099-R, box 1) on line 4b; don’t make an entry on line 4a.