Roth 401(k) distributions will be on Form 1040 lines 5a and 5b, not 4a and 4b. The code-B Form 1099-R will have the taxable amount in box 2a and TurboTax will treat that amount as the taxable amount to be included on line 5b. The entire gross amount from box 1 will be included on line 5a.
