Good point. Desktop TurboTax will include a line 4b explanation in the printed tax return if one is created by clicking the Explain button on line 4b of TurboTax's 1040/1040SR Worksheet. As far as I know TurboTax will include this in the e-filed tax return if it's present in the printed tax return. I don't think that there is any way to create an ad hoc line 4b explanation in online TurboTax; online TurboTax only asks for a line 4b explanation statement for other specific circumstances.
