Does the 2018 Form 1099-R with amounts in boxes 2a and 4 also have code P in box 7? If not, what is the code in box 7? If it has code P, it's rather unusual to have tax withholding on a code P Form 1099-R.
There is little reason to amend the 2017 tax return to include the code P 2018 Form 1099-R that has nothing in boxes 2a and 4.
Any 2018 Form 1099-R that has an amount in box 4 must be included on your 2018 tax return. Any 2018 Form 1099-R that has code P in box 7 and an amount in box 2a must be included on your 2017 tax return, an amended 2017 tax return if not previously reported. That means that any 2018 Form 1099-R that has code P in box 7 and an amount in box 4 must be included on both your 2017 and 2018 tax returns; the amount in box 2a affects the 2017 tax return and the amount in box 4 affects the 2018 tax return.
