1) If the device is not staying connected and constantly dropping data it could be a hardware issue.
Solution: Change out devices. Try doing an exchange and if you still experience constant drops you may need to look at swapping to a different handset all together (ie from Nexus to razr).
2) 4g has been working fine but lately I am getting no connection, mms fails, and sometimes calls will not connect
Solution: Swap out sim cards.
3) My 4g works great but out of no where it drops off for 2 mins.
But its happening more than twice a day. Call a tech they may need to reset the provisions in your phone to make sure its locking on to the towers. Also as OEMS learn more about the network they are building better radios and more efficient processors to lessen the time it takes to switch.