On eDonkey network, each client is identified by a random value (hopefully unique). That value is choosen the first time the client is run.
Any value will do, but eMule peers use that identificator in their credit system, so if you care about your credits you should store your client_md4 in a safe place and check that you don't lose it when upgrading to a new version.
eMule also started the trend to use special values at fixed places in that value to identify client kind (MagicBytes).
Client identifiers play a much more important role on Overnet network, because of the KaDemlia algorithm.
For this reason, the Overnet support doesn't use the client_md4 parameter, but instead uses a random value that the user cannot modify, see ovmd4.