NSEC4 RDATA Format.

Universal Applicabilty.
One additional byte per NSEC3 record.
NSEC4 records are no longer retrievable except as a side effect of a query or via AXFR (IXFR).
Their existance but not content can be discovered by UPDATE.
Precomputed meta data.
They are in a seperate namespace to the names in the zone.
For the root zone they should be treated as being non-existant for queries other than AXFR and their presence does not impact delegations / wildcard expansion.

Conceptually they are is a seperate database attached to the zones database.