I’ve now tried out Smariek’s quadruple decrease and discovered that it leaves a little eyelet each side of the central stitch when that is knitted. I didn’t want those eyelets, so a bit more experimentation was in order.
I needed a slightly left-leaning quad decrease to match to s1, k2tog, psso and here’s what I’ve come up with:
slip 2 stitches knitwise one at a time
pass first slipped stitch over second slipped stitch
slip the next stitch purlwise (this is the centre stitch)
pass the second stitch on the left needle over the first stitch on the left needle
slip the centre stitch on the right needle back to the left needle
leave the remaining stitch on the right needle (it is already turned)
Now k2tog, psso.
The stitch lies flat, unlike the rather lumpy s2, k3tog, p2sso
It also has the least obvious loop on the right of the stitch and the smallest gap between the stitch and its neighbours.