MD4 (“Message Digest [algorithm] #4”) is best known as the data integrity check standard (a.k.a. “hash”) that inspired modern hashes such as MD5, SHA-1 and SHA-2. MD4 codes are 128-bit numbers and are usually represented in hexadecimal format (e.g., “9508bd6aab48eedec9845415bedfd3ce”).
Use of MD4 in modern file transfer applications is quite rare, but MD4 can be found in rsync applications. A variant of MD4 is also used to tag files in eDonkey/eMule P2P applications.
Although MD4 is considered a “cryptographic quality” integrity check (as specified in RFC 1320), it is not considered a secure hash today because it is possible for an attacker to create bad data that bears the same MD4 code as a set of good data. For this reason, NIST does not allow the use of MD4 in key U.S. Federal Government applications.