As goldman said, what you need is a CA870 shotgun - I have a full metal CA870 Charger myself. Then just swap the shotgun body for the Masterkey body (Full metal!) and plop it on your M4.....easy as pie.
Here's a version of the Masterkey kit from G&P, same one ASCA is selling:
http://www.wgcshop.com/pcart/shopper...srch_masterkey
If you want pics, just search for "masterkey" or "M870 Masterkey".