Was the total tax $20,970?

110 % rule?

That's my guess.  I had one pop up today.  No 2018 ES and enough W-2 w/h to get a $1,300 refund.  The 110% rule still had them paying ES for 2019.  Fortunately it's just one box to check to turn it off.

I usually leave those on the "letter", but explain the issue to my client.  IF they chose to roll the dice & not pay, then owe next year; well, my tush is covered 🙂

These are not what you would compare: "the amount due was $4219 but proseries calculated as $1579 x four"

The Amount Due is a balance; you need to look at Tax Total. Estimates would be computed against what you Estimate, or you see the program using one of the Safe Harbor computations against prior year tax Owed. Not Balance Due.