That purl row will be the full row as no other instruction is given.
The purl row will not count towards the 4 rows because patterns are written where you do things in order, finishing each instruction before moving on.
The 4 rows are to be “worked” which means keep on doing the pattern that you have been doing up to this point. If you had been working stocking stitch for the main fabric then work 4 rows would be knit a row, purl a row, knit a row, purl a row.
It may not be stocking stitch for this part as you say it’s a ribbed pattern, but has the ribbing stopped in the last few rows? Or perhaps its a rib like garter rib or another where the WS row is all purl whilst maintaining the rib pattern of alternating knits and purls on the RS row? (hmm I saw another mentioned the other day, different name, looked like similar pattern, I can’t think of the name just now)
You need to “work” in the same pattern you’ve been using up to here.
Next row purl full row
Row 1 dec 1 each end and work rest of row as normal
Row 2 work as normal
Row 3 this is following 2nd so dec 1 each end and work rest of row as normal
Row 4 work as normal
4 stitches remain
Can you work it out from this or do you need more help?