We will present a preliminary analysis of Advanced Camera for Surveys imaging observations of the (proto-)cluster environment around radio galaxy TN1338-1942 at z=4.1. This structure was discovered by searching for Lya emitting galaxies around luminous, high-redshift radio galaxies in a large VLT program, and is presently the most distant structure of galaxies known. We obtained ACS imaging of this cluster field at g-band (4 orbits), r-band (4 orbits) and i-band (5 orbits) using the Wide Field Camera, as part of the ACS Guaranteed Time Observations program. These observations allow us to 1) study the morphology and colours of the 10 spectroscopically confirmed Lya emitting cluster galaxies, and 2) probe an entirely different population of possible cluster galaxies selected using the Lyman-break technique. We will present and discuss our findings of an anomalously high number of g-band dropouts ( ~ 50 objects having IAB<26 and g-r>1.5), which additionally seem to be strongly clustered towards the radio galaxy. The field around radio galaxy TN1338--1942 provides us with a unique laboratory for the study of structure formation in an overdense region of the early Universe. ACS was developed under NASA contract NAS 5-32865, and this research is supported by NASA grant NAG5-7697. We are grateful for an equipment grant from the Sun Microsystems, Inc.
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