  We present the results of new rest-frame far-IR observations of the z = 4.25
radio galaxy, 8C1435+635, which not only confirm that it contains an enormous
quantity of dust (as first inferred from its mm-wave detection by Ivison 1995)
but also allow the first meaningful constraints to be placed on the mass of
this dust and associated gas. The new measurements consist of: (i) clear
detections of sub-mm continuum emission at 450 and 850um obtained with the new
sub-mm bolometer array, SCUBA, on the JCMT, (ii) continuum upper limits at 350,
750 and 175um obtained with SCUBA and the PHT far-IR camera aboard ISO, and
(iii) a sensitive upper limit on the CO(4-3) line flux obtained with the IRAM
30-m MRT. The resulting rest-frame 33- to 238-um continuum coverage allows us
to deduce that 2E8 Msun of dust at a temperature of 40 +/- 5 K is responsible
for the observed mm/sub-mm emission. Using our CO upper limit, which constrains
M(H2)/M(dust) to <950, we go on to calculate robust limits on the total gas
reserves (H2 + HI) which are thereby constrained to between 4E10 and 1.2E12
Msun. The sub-mm properties of 8C1435+635 are thus strikingly similar to those
of the z = 3.80 radio galaxy, 4C41.17, the only other high-redshift galaxy
detected to date at sub-mm wavelengths whose properties appear not to be
exaggerated by gravitational lensing (Dunlop et al. 1994; Hughes, Dunlop &
Rawlings 1997). The inferred gas masses of both objects are sufficiently large
to suggest that the formative starbursts of massive elliptical galaxies are
still in progress at around z = 4. Observations of complete samples of radio
galaxies spanning a range of redshifts and radio luminosities will be required
to determine if the spectacular far-IR properties of 8C1435+635 and 4C41.17 are
primarily due to their extreme redshifts or their extreme radio luminosities.
