  We present the first results of a deep imaging programme to identify the
system responsible for the Cosmic Microwave Background decrement in the field
towards the z = 3.8 quasar pair PC1643+4631 A&B. Using the prime focus camera
at the William Herschel Telescope we have carried out deep multicolour optical
imaging to search for candidate cluster galaxies at extremely high redshift.
Using UGR colour selection we find the surface density of z > 3 Lyman-break
galaxy candidates is at least as great as that found in the field of the z =
3.1 structure discovered by Steidel et al. (1998), and may be somewhat greater.
