We exploit wide-field Ly$alpha$ imaging with Subaru to probe the environment around TN J1338-1942, a powerful radio galaxy with a >100 kpc Ly$alpha$ halo at z=4.11. We used a sample of Ly$alpha$ emitters (LAEs) down to $log(L_{rm Lyalpha} [erg, s^{-1}])sim 42.8$ to measure the galaxy density around TNJ1338, compared to a control sample from a blank field taken with the same instrument. We found that TNJ1338 resides in a region with a peak overdensity of $delta_{rm LAE}=2.8pm 0.5$ on scales of $8, h^{-1}rm Mpc$ (on the sky) and $112, h^{-1}rm Mpc$ (line of sight) in comoving coordinates. Adjacent to this overdensity, we found a strong underdensity where virtually no LAEs are detected. We used a semi-analytical model of LAEs derived from the Millennium Simulation to compare our results with theoretical predictions. While the theoretical density distribution is consistent with the blank field, overdense regions such as that around TNJ1338 are very rare, with a number density of $6.4times 10^{-8}rm Mpc^{-3}$ (comoving), corresponding to the densest < 0.4 percentile at $zsimeq 4.1$. We also found that the Ly$alpha$ luminosity function in the TNJ1338 field differs from that in the blank field: the number of bright LAEs ($log(L_{rm Lyalpha}[erg,s^{-1}]) gtrsim 43.3$) is enhanced, while the number of fainter LAEs is relatively suppressed. These results suggest that some powerful radio galaxies associated with Ly$alpha$ nebulae reside in extreme overdensities on $sim 3$--$6, rm Mpc$ scales, where star-formation and AGN activity may be enhanced via frequent galaxy mergers or high rates of gas accretion from the surroundings.
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