The current instructions to enter in the new W4 information for section 4 requires you to aggregate lines 4a, 4b, and 4c. Lines 4a and 4b are taxable income amounts, and 4c is a tax amount. Combining values into the Additional Withholding box described in the instructions would result in incorrect tax calculation.

MS support has a workaround to manually calculate the tax amount for lines 4a and 4b outside of GP then add to line 4c and put into the additional withholding field. This is not a sustainable practice as end users would have to calculate this for each person and each pay run.

I would suggest adding in additional fields to put in the values that would then let GP calculate the tax amount based on the employee setup and pay structure.