The physical parameters of the GJ 3998 planetary system against the literature. Centre: Mass-flux and minimum mass-period diagrams of exoplanets with well-established masses and periods from Akeson et al. (2013) (red squares for M-dwarf hosts, blue circles otherwise). Against those, we plot the derived posteriors of our best model (d4; black error bars) using stellar properties from Maldonado et al. (2020). Reported uncertainties reflect the 16^th and the 84^th percentiles of our posteriors. Top and right sides: one-dimensional histograms of well-established planets from Akeson et al. (2013) (solid red lines for M-dwarf hosts, dashed blue lines otherwise). The locations of GJ 3998 b, GJ 3998 c and GJ 3998 d are marked by dashed black lines.