Not quite L4D, but the closest any game has come to it
B4B doesn't quite hit the same heights as L4D, but it comes much closer than any other game does to scratching the same itch.
While some of the systems like cards can seem intimidating at first, and might act as a barrier vs the pick up and play simplicity of L4D, the learning curve isn't overly steep, and once that hurdle is passed there is a lot of fun to be had here.
Difficulty is also handled very well. Starting from the lowest difficulty in co-op, jumps to each new difficulty level felt significant, but not overwhelming with some practice. Definitely recommend this title for some co-op Zombie killing action.