Korg DW8000, Mono/Poly, MS-20...

Hi!
I just purchased a DW8000. If I hold four notes simultaneously, then
the fifth note played turns off the first note I already played (in
POLY1 or POLY2 mode). It seems that my DW8000 has only 4 notes
polyphony. Is this possible? Or am I just dreaming? Any ideas?

Thanks, Jussi