The formation and evolution of galaxies is regulated by the exchange of gas with the surrounding large-scale structures on circum- and intergalactic scales, yet little is known about the complex processes shaping the cycle of baryons in and out of galaxies. In this work, we present a multiline study of the gas surrounding a z ≈ 3.66 quasar known to host one of the brightest Ly α nebulae at high redshift, MUSE Quasar Nebula 04 (MQN04). By combining a high-resolution MUSE detection of nonresonant He II emission with a precise measurement of the redshift of the quasar host via the ALMA CO(4–3) line, we study the kinematics of the cool ionized gas down to #x2248; 1 kpc from the quasar. The MUSE observations reveal complex clumpy structures as well as diffuse emission extended over ≈100 kpc and blueshifted by ≈0 − 800 km s ⁻¹ relative to the quasar systemic redshift, suggesting that the circumgalactic medium is highly asymmetric. The analysis of the He II /Ly α line ratio, and the presence of a low-column density (≈ 10 ^14.6 cm ⁻² ) H I absorber along the quasar sightline suggests that MQN04 resides in a highly ionized medium. This is also supported by the gas kinematics, which, except in the most central region, shows consistent velocity shifts across the different tracers, indicative of relatively weak radiative transfer effects. Based on its morphology and kinematics, we conclude that the extended He II emission may arise from merger-driven tidal stripping or inflows of gas illuminated by the quasar radiation. On comoving megaparsec scales, we discover a large concentration ( δ ≈ 41) of star-forming galaxies lying within |Δ v _QSO | ≲ 1000 km s ⁻¹ from the quasar. MQN04 is therefore one of the most overdense environments discovered at this epoch.

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