Well? Any CEL's now? 4,000 rpm isn't a magic number for a clogged cat, but it will feel like it just runs out of power to rev any higher. The key charicteristic of a clogged cat is tons of sound and fury with no power
If it's an ECU issue (ie. limp mode, VTEC solenoid, VTEC pressure sensor, emissions problem) then the ECU will set a rev limiter at 4,000 rpm.