Just FYI, I just got back from Rifle Carbine School for my PD. I am now carrying in my squad a Rock Rive Arms M4 Tactical Entry LEO with the RRA 6 Position Collapsible Stock. It is nice and small/compact with the stock collapsed for carrying and room entry. I have also found it to work great with the stock extended out when longer distance shooting is required.
My rifle came with two sling attachments up front, one on the side and one on the bottom. I bought a GG&G Rear Sling Thing for the RRA rear stock and a GG&G Receiver Sling Plate. So, with the stock bottom mounted sling attachment I have 3 sling attachment points in the rear. With the front two and the rear three, I can adjust the sling indefinately. Also, for slings, may I HIGHLY suggest the Eagle Tactical 3 Point Sling. Simply awesome.
So basically, I would highly suggest the M4. Out dept. two K9 handlers both carry,use M4s and they seemthe like them alot.