You are going to be making a thumb gusset which is a little upside down pyramid or triangle that will start with these 4 stitches and end up with 14 stitches in it. This gusset will become the thumb.
I noticed that it says to work to the marker. In the notes part at the top of the pattern, it said in one section called HELPFUL that you might like to place a marker on each side of the cable on the back of the hand (I don’t think I would, it is easy to see) and you might like one where you change from the chart to the St st, that is a very easy to see transition, but that is the marker they are talking about here.
You want to work across the back of the hand and as it is written you are to put the first kfb of the gusset in that first st in the St st part. If you have a marker there (or not and just remember that the first St st is the beginning of the gusset), slip the marker and K1fb in the first St st, then do that again in the following stitch, slide a marker onto the RH needle. You have 4 stitches (the gusset) between markers (or the beg of the St st and the marker). Finish knitting the row.
Then you purl back across all the stitches.
Next row says:
(RS) Work across to marker; k1-f/b in st after marker, knit across to last 2 sts before second marker, k1-f/b, k1; work to end—6 sts between Thumb Gusset markers.
So work to the beginning of the St st and slip the marker if you placed one there, then K1fb in the next stitch, knit until you have 2 sts before the marker at the far side of the gusset and K1fb in that stitch, k1, slip the marker. Knit across the row. (6 sts in gusset).
You keep repeating that gusset increase row and the even purl row until you have 14 sts between the markers and the last of the row.
On the other mitt you will work the back of the hand and then knit across the St st palm part until you have two stitches left to work the gusset part in. Then you would place a marker on the RH needle and do the Kfb in the last 2 sts. You won’t really have a marker at the far end. On subsequent rows you would work until you have 14 sts between that gusset marker and the end of the row.