Md4 (Message Digest 4) is a cryptographic hash function created by Ronald Rivest in 1990. This algorithm takes as input a string and makes a hash from it. This hash is 128 bits, 32 caracters long. Because of security problems, Md4 was abandoned for its little brother, Md5 . The reason for that is that one can easily attack Md4 with collisions, in a few milliseconds. The Md5 is more secure, because you need 2^64 operations (anniversary attack) to make a collision (even though Md5 is not secure enough). Microsoft's NTLM hash use Md4 with few differences. You shouldn't use Md4 as a cryptographic function to store critical data, since it's very easy to decrypt. This website allows you to decrypt your md4 hashes if you're lucky of course. Our online database contains more than 1.000.000.000 Md4 hash, coming from all the wordlist I was able to find on the internet. I then computed for days to enlarge it and to make it relevant and unique, in order to help you decrypt your Md4 hashes.
You shouldn't use Md4 to store your passwords, you would better be using Sha2 functions, and use a salt, which is a little (or not) string that you add to your user's password to make it way more difficult to bruteforce or to find using rainbow tables such as this website which provides you a way to decode Md4 hashes.