The MMM is currently haveing some issues with the internal BEC. So you might want to hold off on that untill they get it sorted out. The Mamba Max is a good low cost alternative for 4s. Many people have run and are running that esc with great sucess on 4s using an external BEC and a 25mm fan attached to the heatsink. Its a pretty well proven setup.
Yes, the voltage thing is the battery. 4s means 4 cells in series (14.8v) 5000 mah is the capacity of the pack. The Polyquest packs are very good quality. Other good quality options would be a the NeuEnergy or Flightpower. I am persoanlly not a fan of the Maxamps packs. Overpriced and under performing IMO. Check out the RCM store.
http://www.rc-monster.com/batteries.php There are also other options available out there if you look around.