Critter is correct. The change last year caused massive amounts of confusion, but TurboTax is correctly completing line 4 according to the (confused) IRS instructions.
The problem stems from rollovers. According to the IRS instructions (see page 28 and following at https://www.irs.gov/pub/irs-pdf/i1040gi.pdf), you enter some distribution amounts in 4a and you don't enter others, even though you enter all taxable distributions in 4b. This instruction can make 4b, the taxable amount, larger than 4a (which is supposedly the total amount of distributions).
As Critter says, don't worry about it. TurboTax is doing it according to the IRS instructions, and the IRS is only going to worry about 4b anyway.
