The Roland VP-550/770 are both Vocal Designers. They use real voice imput for generating a choir. I think you can't compare this technology to stand alone voices. Of course, the Vocal processor on the Tyros4 is much worse than the just mentioned devices. I think the stand alone choir voices on the T4 are better than on the old and recent Roland devices (compared to the Fantom-G8 and G-70 i had for many years).

Best Regards,

Joost