Baby bib question

I am knitting a baby bib that has a separate piece that I attach to the front. It is a tie that has 24 rows. On the 19th row it says to knit which I have 4 stitches on my needle. The 20th row says knit 2tog knit 2tog which I understand and leaves with 2 stitches. The part I am confused about is the 21st row it says knit 1 cast on 4 knit 1 which I should have 6 stitches. I am totally not understanding how to complete this row, all the other rows I understand. I am very frustrated with this row due to the fact it only consists of 24 rows and the project would be complete. Please help me complete this so I can get this sweet project completed and I can rest my brain. Thank you

Hi and welcome!
You may be making a hole for a button.
Knit one, switch the needles in your hands and use a knit or cable cast on to cast on 4sts, then switch the needles back and continue with the remaining knit one.
This video may help although you don’t have the line of cast off sts that are shown.

Thank you for the response and your welcoming. This information is very helpful. I will check out the video. Thanks again.