Well it all depends on what you like best. I will sate the states for both weapons and you make your choice.
Tavor Assault Rifle - 21
Max Damage: 35
Min Damage: 25
This means that the maximum damage that the TAR - 21 can do, is 35 hp points. Now you may be wondering why it only takes 2 - 3 bullets to kill with this gun, this is because certain parts of the body have damage multipliers. For example the body has a 1.5 damage multiplier.
Drop Off Damage: 10
For every 25 feet, how much damage drops off per bullet (This is an average)
M4A1
Max: 30
Min: 20
Drop off: 10
You make the choice.
Personally I like the TAR way better than the M4, but that is just my opinion.