JB users 8TB internal.
External 8TB using official extended HDD.
JB users using APP2USB
Internal 8TB
Not sure if there is a limit but external could use up-to 20TB if you wanted but are limited to 1000 apps on internal and 1000 apps external.
um i dont think this is entirely true i believe the very first two model numbers for the original fat ps4 can only do 4 tb internal from my understanding correct me if im wrong