The coefficients $c_0$ (left), representing the linear contribution, and $c_1$ (right), quantifying the nonlinear dependence on $v_2^2$, from fits of Eq.~\eqref{eq:v4fit} to the $v_2\{2\}$--$v_4\{2\}$ correlations shown in Fig.~\ref{fig:v2v4cor} for Pb--Pb (solid red markers) and Xe--Xe (open red markers) collisions as a function of the collision centrality. Xe--Xe points are slightly shifted horizontally for better visibility. Measurements from the ATLAS Collaboration in Ref.~ (black points) are shown for charged particles in the kinematic range $0.5 p_\mathrm{T} 2.0~\text{GeV}/c$ and $|\eta| 2.5$. Error bars (boxes) indicate statistical (systematic) uncertainties.