  We present deep optical imaging of the z=4.1 radio galaxy TN J1338--1942
obtained using the ACS on-board HST. The radio galaxy is known to reside within
a large galaxy overdensity (both in physical extent and density contrast).
There is good evidence that this `protocluster' region is the progenitor of a
present-day rich galaxy cluster. TN J1338 is the dominant galaxy in the
protocluster, in terms of size and luminosity and therefore seems destined to
evolve into the brightest cluster galaxy. The high spatial-resolution ACS
images reveal several kpc-scale features within and around the radio galaxy.
The continuum light is aligned with the radio axis and is resolved into two
clumps in the i-band and z-band bands. These components have luminosities ~10^9
L_sun and sizes of a few kpc. The estimated star-formation rate for the whole
radio galaxy is ~200 M_sun/yr. A simple model in which the jet has triggered
star-formation in these continuum knots is consistent with the available data.
An unusual feature is seen in Lyman-alpha emission. A wedge-shaped extension
emanates from the radio galaxy perpendicularly to the radio axis. This `wedge'
naturally connects to the surrounding, asymmetric, large-scale (~100 kpc)
Lyman-alpha halo. We posit that the wedge is a starburst-driven superwind,
associated with the first major epoch of formation of the brightest cluster
galaxy. The shock and wedge are examples of feedback processes due to both AGN
and star-formation in the earliest stages of massive galaxy formation.
