I have been recently doing quite allot of work with File Replication Service. The File Replication service is used to replicate the contents of your sysvol folder e.g. Group policies and Scripts. The difference between FRS and Active Directory Replication is that the FRS service is a multi-master replication service, that replicates only whole files,no just the deltas.Therefore the entire file is replicated even if you change only a single byte in the file. So where does D2 and D4 come into the equation.
D2 is a registry key entry which is used as a FRS non-authoritative restore which is used to bring the local FRS replica set back up to date with its partners. The is completed by comparing all the local files in the replica with the ones of its upstream neighbour. The D2 element refers to the BURFLAG (Backup and Restore Flag) registry key. When this registry key is populated with a D2 entry this directs the local replica to re-initialize with its upstream partner.
D4 is a registry key as above which is used by FRS to signify an Authoritative restore. This makes the current replica set of existing files authoritative for all the members of this replica set. The FRS replication service MUST be stopped on all replication partners of a replica set before carrying out this procedure otherwise you will initiate a morphed condition. (More about morphing tomorrow). IMPORTANT you must only perform the two procedures above by first consulting your support provider, as it is often a last resort solution. Also thoroughly read the following KB article http://support.Microsoft.com/kb/315457/.