The line numbers have changed but the rule has not and TurboTax has followed this rule for more than 9 years.
If you examine your past tax returns, you'll likely find that the amount on line 4b (or its equivalent) is lower than the amount on line 4a, either due to some amount being rolled over or to having had basis in nondeductible traditional IRA contribution reducing the taxable amount, calculated on Form 8606.
