The correlation between $v_4\{2\}$ and $v_2\{2\}$ using $q_2^\mathrm{V0C}$ selection (left) and $v_4\{2\}$ and $v_3\{2\}$ using $q_3^\mathrm{V0C}$ selection (right) in Pb--Pb collisions. The open black points, connected by a black line for visibility, show the unbiased correlation. The $q_n$-differential correlations are shown by the full markers in each centrality interval. Error bars (boxes) indicate statistical (systematic) uncertainties.