This is where it gets interesting -lol.
Take 2 of your quilted panels and place them together like this -
Sew along the pinned edge matching seams. When folded back it should look like this-
Sew the other 2 panels in exactly the same way.
Sew these 2 pieces together so they look like this -
Now fold up one piece matching seams again - and sew along this edge. The top edges will not meet - one piece will be longer than the other by 2 squares.
Stitch the other 3 edges the same way !
You should now have this -
One more extra thing you can do at this point is using a small sharp scissors is trim the wadding from the seam allowances.
This will help the bag keep it's shape. I also give it a little press with the steam iron where the panels have been joined together.