  We present sensitive 850 micron SCUBA photometry of a statistically-complete
sample of six of the most luminous, z>4 radio-quiet quasars, reaching noise
levels comparable with the deep blank sky surveys. One quasar (BR2237-0607;
z=4.55) is detected at 850micron with a flux of 5.0mJy (4.5sigma), whilst a
second (BR0019-1522; z=4.52) has a detection at the 2 sigma level. When
combined with our previous millimetre measurements of z>4 quasars, we find that
there is a large range (5--10) in far infrared (FIR) luminosity at fixed UV
luminosity, and that the typical quasar has a L_FIR and mass of cool (50K) dust
similar to that of the archetyepal low redshift (z=0.018) ultraluminous IRAS}
galaxy Arp220. If one assumes a fiducial FIR luminosity of 5x10^12 Lsun for for
all quasars with M_B<-23, we find that around 15 per cent of the sources in the
SCUBA deep surveys could be classical broad-lined radio-quiet AGN. Thus if one
considers the observed ratio of Seyfert II to Seyfert I galaxies at low
redshift and any contribution from totally optically obscured AGN, a
significant fraction of the SCUBA source population will harbour AGN and hence
the inferred star formation rates from submm fluxes may be overestimated if the
active nuclei are bolometrically dominant or the IMF is top heavy.
