These are the differences taken from specs in manuals:

Effects:
Pa4x:
Accompaniment/Song: 4 insert effects; 3 master effects (148 types)
Final mastering effects by Waves Audio®:
Waves Audio MAXX Suite, including MaxxEQ™, MaxxBass™, MaxxTreble™, MaxxStereo™, MaxxVolume™
Voice Processor Technology by TC•Helicon®:
Four-part Harmonizer, Double, μMod, Reverb, Delay, Filter, HardTune, Pitch Correction; plus Compressor, EQ, Gate (Adaptive Mic Setting on the Mic input)

Pa1000:
Accompaniment/Song: 2 insert effects; 2 master effects (148 types)
Final mastering effects: Limiter, Master EQ
Voice Processor: EQ, Compressor, Gate, DeEsser, 3-voice Harmonizer (or Double), 2 x FX. Harmonizer, Double, DeEsser: Technology by TC•Helicon®


Sounds:
Pa4x: More than 1900 factory sounds
Pa1000: More than 1750 factory sounds


Sampling:
Pa4x: 400MB (compressed)
Pa1000: 300MB (compressed)


Styles:
Pa4x: More than 550 factory styles, 1248 user locations
Pa1000: More than 430 factory styles, 1152 user locations


Keyboard set library:
Pa4x: More than 460 factory, 1056 user locations
Pa1000: More than 300 factory, 864 user locations


Obviously there are also physical differences, missing controls and connections, speakers added.....