1) The OEM C-Stock. No Benelli M4 is complete without it. Fully extended position for the ghost ring sights, middle position for small reflex sights (i.e. Aimpoint Micro, Trijicon RMR, etc...) and fully collapsed for storage.
2) The Urbino stock I purchased directly from Mesa Tactical (Mfg.# 91470) had a problem with the pistol grip rubber sleeve. It came loose after just minor handling once installed on my shotgun. I returned it for a full refund. The cheek riser is quite solid and the Limbsaver recoil pad is pretty comfortable. If you insist on going with an EoTech sight - too tall and bulky IMO for the M4 - you'll find the cheek riser feature useful.
3) Lowest price I could find from a quick Google search was from New Frontier Armory @ $132.60 They have a sizeable stock, but I've never purchased anything from them so I can't provide any feedback. MidwayUSA and Brownells sell them @ $149.99 but are out of stock at the moment.
4) Try GunBroker, they pop up from time to time at decent prices. If a smooth transaction and super fast shipping is what you want, our fellow member heckler&kochp2000 is the one you need to contact for this item.