Here's your answer: Pixel Force's Left 4 Dead "de-make."
Go here to download the game.
Also, Eric David Ruth, the (one and only) man behind this retro re-imagining of Valve's "zombie-killing masterpiece" (Ruth's words, not mine), answers quite a few questions about the title here and here.
I've only played the game for a few minutes so far, but what I have played has really impressed me. My only complaint: That the game isn't an actual NES ROM.