Look at RFC 7531. XDR definitions for NFSv4 exists, and they can also be used to generate C bindings if you want.
/// /*
/// * Remote file service routines
/// */
/// program NFS4_PROGRAM {
/// version NFS_V4 {
/// void
/// NFSPROC4_NULL(void) = 0;
///
/// COMPOUND4res
/// NFSPROC4_COMPOUND(COMPOUND4args) = 1;
///
/// } = 4;
/// } = 100003;