Don't understand what the pattern is saying

I am working on a baby cardigan sweater by Debbie Bliss. I am on the sleeve portion and almost done. I am not quite sure what she means.
[COLOR=blue]Dec one st at each end of next and 3 foll alt rows. 40 sts. cast off.[/COLOR]
[COLOR=black]I am working in st st and I have 48 sts on the needle. I understand the Dec one st but what I am not getting is the 3 foll alt rows. Do i dec every one and 3rd row (which will only be on the k side) or every 3 rows until I get 40sts. I am racking my brain. I know this is simple but I just can’t seem to grasp it tonight.[/COLOR]
Thanks, dvlwoman

Here’s what you do:

The first decrease row we’ll call “row 1”. So. . .

Row 1-- dec 1 st at each end.
Row 2-- purl the 46 sts.
Row 3-- dec 1 st at each end.
Row 4-- purl the 44 sts.
Row 5-- dec 1 st at each end.
Row 6-- purl the 42 sts.
Row 7-- dec 1 st at each end.
Row 8-- purl the 40 sts.

Make sense?

Okay, alt rows means every other row, so you would dec on the next row, then every other row after that for 3 times. There would be 4 dec rows in all and 40sts on the needles.