SSSP (slip, slip, slip, purl) is a double decrease and a mirror image of P3tog, decreasing your total number of stitches by two.
How to work a SSSP decrease
So I have cast on 10 stitches and worked a few rows in stocking stitch, on the next purl row the instructions are :
Purl to last 4 stitches, SSSP, P1. (8 sts)
I have purled to the last 4 stitches of the row and am now ready to work SSSP:
Step 1 – Slip 1 stitch knitwise from the left hand needle to the right hand needle, slip the next 2 stitches on the left hand needle to the right hand needle (one at a time) in the same way as the previous stitch.
Step 2 – Slip the 3 stitches back onto the left hand needle, by inserting the left hand needle from left to right.
Step 3 – Insert the right hand needle through the back loops of the 3 slipped stitches (from left to right).
Step 4 – Pass the yarn around the back, up and over the front of the right hand needle (as you would if you were purling a normal stitch) .
Step 5 – Pull the yarn through the three stitches, slipping them off the left hand needle.
A double descrease completed.