If your Roth 1099R is coded Q, the IRS should be able to figure this out all right by disregarding the Roth 1099R, and your 4a should then equal the non Roth 1099R amounts.
The combining of prior lines 15 and 16 was not a good idea, and the IRS again separated these lines on the 2019 1040 to have IRAs reported separately from qualified plan distributions. IRS matching for 2018 will probably be compromised, but in most cases the IRS can probably figure it out.
May be a little tougher for them if your Roth 1099R is coded T instead of Q. Some IRA custodians will not provide the Q code. Even with the T code, you could easily enough wait for the IRS to contact you since your tax liability is not affected. Or you could file the 1040X if you wanted to further reduce the chance of getting any IRS inquiry. But if it were me, I wouldn’t do the 1040X.
Which year’s instructions are you looking at? In 2018, there was no line 4c or 4d, the Form 1040 design required that all deferred accounts be reported on 4a and 4b, so you had no choice. Then for 2019, the IRS realized that combining all these plans on a single line made it more difficult to track distributions, and in 2019 they again separated the lines in 4a,b,c, and d. You might be looking at the 2019 1040 Inst.
The TT worksheet shows the detail, and the taxable amount is correct. I looked at the IRS instructions and could not determine if the gross amounts for these items should show on line 4a or not- just seems off particularly since the line description includes them