The armhole edge is the edge where you are binding off those 4sts. So you'll BO 4 then finish that row. Turn your work and work back towards the armhole edge, and once you get to the edge you will need to decrease 2 stitches. You can do this by doing two K2togs in the last 4 stitches of the row OR you can do a double decrease by doing a sk2p in which you will slip on stitch, k2tog, then pass the slipped stitch over.
So that takes care of the 'dec at armhole edge 2sts ONCE', so to do the last part of your instruction you will turn your work around again and decrease 1 stitch at the armhole edge (the beginning of the row), work to the end of the row, turn and work back to the armhole edge and decrease 1 stitch again. That's the 'dec 1st TWICE'.
Hope this helps.