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Description
We present the first near-infrared interferometric data of the broad line region of a QSO at z=4. GRAVITY+ detects a differential phase signal tracing the spatially resolved kinematics for both the H$\beta$ and H$\gamma$ lines. Fitting these lines simultaneously shows that 80% of the HI line emission in the broad line region originates in an outflow with a velocity up to $10^4$ km/s, oriented so that our line of sight is along an edge of the conical structure. The model is also able to qualitatively match the outflowing CIV line profile and the systemic OI fluorescent emission. The derived black hole mass of 8e8 Msun is an order of magnitude lower than that inferred from various single epoch scaling relations, and implies that the accretion is super-Eddington. With reference to recent simulations, the data suggest that this QSO is emitting close to its radiative limit in a regime where strong outflows are expected around a polar conical region.