MD4 is a message-digest algorithm that produce 128-bit fingerprints. Description and source code for the algorithm can be found in RFC 1320. MD4 is now considered as insecure/broken. A common alternative is SHA-1 although it is also under attack.
Used in eDonkey2000 to identify files. That makes an organized effort to poison the network something doable.
TakeDown.NET -> “MD4”