Explanation
The rpc.idmapd service is a new component of NFSv4 that does not exist in NFSv3. It is responsible for mapping user and group IDs between the NFS client and server, using string names instead of numeric values.
This allows for better security and compatibility across different systems. The rpc.idmapd service runs on both the NFS client and server and communicates with each other using the NFSv4 protocol. The other services, rpc.statd, nfsd, and rpc.mountd, are common to both NFSv3 and NFSv4, although they have some differences in functionality and behavior. References: 1, 2, 3