Far-IR/sub-mm photometry of high-redshift objects probes their rest-frame mid-
and far-IR emission, constraining T(dust), M(dust), L(far-IR), and the SFR at
early epochs. Currently, rest-frame far-IR data exists only for a handful of
radio galaxies and QSOs at z ~ 4, for a handful of lensed QSOs at moderate
redshifts, and for the local examples detected by IRAS.  The detections
reported to date have only three things in common: the S/N levels are low; the
calibration is uncertain; and the lack of rest-frame mid-IR data (i.e. PHT-C
measurements) means that temperature constraints are extremely poor. Here, we
propose to obtain PHT-C data at 90 and 160 microns for 55 QSOs with redshifts
in the range 3.7 < z < 4.4, tightly constraining dust properties and
determining the QSO far-IR luminosity function in the early Universe. Our
redshift criterion is commensurate with our long-term aim of obtaining high
spatial resolution CO(1-0) and CO(2-1) maps with the 0.7- and 1.3-cm receivers
of the upgraded VLA. Of the 55 QSOs in our sample (which contains all known
QSOs at these redshifts), 20 have already been observed to 3-sigma levels of 5
mJy or better at 1250 microns with early ground-based mm-wave bolometers (e.g.
Ivison 1995) and 5 have been confirmed (with S/N > 5) as being extremely
luminous in the rest-frame far-IR, suggestive of significant flux density
levels (~50-100 mJy) in the PHT-C filters. We have a parallel programme to
observe this sample of QSOs at wavelengths beyond 350 microns with the SCUBA
sub-mm camera on JCMT: the combined ISO, SCUBA and VLA measurements will be
crucial for our understanding of the evolution of the gas and dust content and
the star-formation histories of QSOs.