Battlefield 4 isn't the only new first person shooter to see the addition of a lean feature, which allows players to lean out from behind cover to take out their enemies while barely revealing their position.
Call of Duty: Ghosts also has a lean function, but unlike its implementation in Battlefield 4, which clearly shows player characters as they reveal themselves from cover to lean out and take a shot, Ghosts' lean function does not appear to have a leaning animation and players leaning from cover remain invisible to their opponents.
The problem is clearly depicted in this Vine below.
Update:
This is clearly a visual bug as the leaning character can be spotted, at one point, having his gun stick out and clip through the wall.
Needless to say, it's going to be fixed, as confirmed by Infinity Ward's Tina Palacios, who tweeted, "Nice footage of a stolen game that isn't supporting our work. That is going to be fixed in TU1," admonishing the person who posted the footage while simultaneously confirming that it's an issue the developers are aware of.