In order to unlock the MTAR 21- Carbine in Battlefield 4 you need to complete the Multi Talent Assignment. The requirements for completing it are:
Reach Rank 10
Have the China Rising DLC
Then in one round you must get
A kill with a sniper rifle, light machine gun, assault rifle and a grenade.
The first three kill requirements aren’t hard to do, even though I am a bad sniper. However getting a grenade kill is not always easy to do. There is a lot of luck in the grenade. The best bet is to go to a level where there are often a concentration of enemies, and hope some have already taken damage. Operation Locker of Metro are good examples. The next tip if you are waiting to get a grenade kill is to run as the support class so you can replenish your supply of grenades, even though it can take some time.
After you have completed the Multi Talent Assignment, you will be rewarded with a digital version of the Israeli made MTAR – 21 carbine. This carbine has a fast rate of fire, blasting away at a rate of 900 rounds per minute. That statistic alone makes it a great close quarter combat weapon in game, but its other stats make it a poor choice for medium to long range.