SSSK (slip, slip, slip, knit) is a double decrease and mirror image of K3tog, decreasing your total number of stitches by two.
How to work a SSSK decrease
So I have cast on 10 stitches and worked a few rows in stocking stitch, on the next knit row the instructions are :
K1, SSSK, knit to end. (8 sts)
I have knit the first stitch of the row and am now ready to work SSSK:
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 – Place the tip of the left hand needle through the front loops of the three stitches that were slipped onto the right hand needle.
Step 3 – Pass the yarn around the back, up and over the front of the right hand needle (as you would if you were knitting a normal stitch).
Step 4 – Pull the yarn through the three stitches, slipping them off the left hand needle.
A double descrease completed.