When entering the 2017 Form 1099-R into 2016 TurboTax, I would probably omit the amount from box 4. Entering the tax withholding in box 4 may cause TurboTax to try to include the made-up 2017 Form 1099-R in your e-filing (although it shouldn't since the tax withholding does not apply to 2016). TurboTax used to have a bug where it credited the tax withholding to *both* years, so this area has not been without its challenges.
