An MD4 hash is a unique value given as a result of a mathematical expression following the MD4 algorithm which claims to ensure data integrity.
It provides a 128-bit (16 bytes, 32 hex chars) message digest from the data input (the input may be of any length) that is claimed to be unique.
NOTE : the hash used in ed2k protocol URLs in NOT MD4 !!! It is similar but not the same. Above certain filesize (the exact value is documented somewhere) the MD4 and the ed2k hash for the same data are not the same. Anybody can check this by computing a hash of any file greater than 15 megabytes using any hash tool, like HashCalc, fsum etc.