Hα line transmission spectrum analysis of MASCARA-4 b. Panel a: residuals after removing the master out-of-transit spectrum and binning in orbital phase by 0.002, containing the stellar RM+CLV effects and the planetary absorption. Panel b: best-fit RM+CLV model to the data in panel a. Panel c: difference between the data (in panel a) and the RM+ CLV model (in panel b) to isolate the planetary absorption signal, as traced by the slanted white line. Panel d: transmission spectrum in grey obtained by average-combining the in-transit residuals in panel a (without the correction of RM+CLV effects) in the planet rest frame. The black dashed line is the RM+ CLV model in panel b combined in the planet rest frame. Panel e: transmission spectrum in grey obtained by average-combining the cleaned residuals in panel c in the planet rest frame. The black points show the transmission spectrum binned every 15 data points. The red dotted line shows the best-fit Gaussian profile to the planetary absorption feature of Hα.