What is the RNP requirement within the KZAK FIR?
To be exact RNAV 10 is required when operating in the KZAK FIR. This enables 50NM longitudinal and lateral separation between aircrafts.

Unfortunately, the requirement might be designated RNP 10 due to legacy reasons. The difference between RNAV and RNP is that RNAV requires no onboard performance monitoring and alerting capability, while, among other things, RNP does.

RNP 4 is optional to allow closer 30NM longitudinal and lateral separation between RNP 4 capable/authorized aircraft thus increasing the airspace capacity.

Sources:


ICAO Doc 9613 - Performance-based Navigation (PBN) Manual
ICAO Doc 8168 - Procedures for Air Navigation Services — Aircraft Operations, Volume I (PANS-OPS Vol I)