The $4,900 balance is NOT 100% post-tax. Your 2018 Roth conversion is partly taxable and partly nontaxable. I think that you are forgetting that part of the basis from your 2017 $5,500 contribution applied to your 2017 Roth conversion done prior to the 2017 contribution, causing less than $5,500 of basis to carry forward to 2018. With the sum of the basis carried forward to 2018 plus the $5,500 of basis added by your 2018 contribution being less than the sum of your $5,500 2018 Roth conversion and $4,900 year-end value, only some of the basis applies to your 2018 Roth conversion, leaving some amount of basis in your traditional IRAs along with the $4,900 year-end balance.
If you care to share the amount of your 2017 Roth conversion, I can tell you what you should be seeing on your 2017 and 2018 Forms 8606.
