Players have been experiencing a tough time in trying to kill a fallen soldier. It gets even tougher when shooting, throwing a grenade or even launching an RPG at a half-dead player only to see it experiencing negative effect.
This is actually due to a bug that somewhat acts like a guardian angel on players who are waiting to be revived. At that state, even a tank shell will do no effect to the player. The only way he or she can be killed is if the revive countdown timer hits zero. Of course, this will not happen if some medic came and save the fallen one.
To avoid this scenario, it is best to take out other players with explosives. Using those will prevent the enemies from going into that state of immunity and kill him or her straight instantly.
The downside is that, DICE has yet to acknowledge the bug. Battlefield 4 players better be loud and force the game developer to roll out the fixes.