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.