As implied in Critter#2's answer, line 15c of Form 8606 only reports the taxable amount of distributions from traditional IRAs not converted to Roth. Line 4b of 2018 Form 1040 reports the taxable amounts of distributions from *all* types of retirement accounts and from Roth conversions, so only if you had no retirement distributions taxable in 2018 other than distributions from traditional IRAs (not including Roth conversions) will line 4b of Form 1040 equal line 15c of Form 8606. (The taxable amount of conversions from a traditional IRA to a Roth IRA appear separately on Form 8606 line 18.)
Also, if you are married filing jointly and both of you have taxable traditional IRA distributions not converted to Roth, the amount on line 4b will include the amounts from line 15c of both Forms 8606.
The amount on Form 1040 line 4b will never be less than the amount of Form 8606 line 15c, but it certainly can be more due to taxable distributions from other types of retirement accounts or from Roth conversions.
