GWB amplitude distributions predicted by the RSG15 models. The thin-dashed yellow line is for the full set of models in RSG15, whereas the thick-dashed orange line is for the subset considered here. The solid blue line is the distribution predicted by the 108 down-selected sample used in the analysis. The vertical line marks the median value of A at f₀ = 1 yr⁻¹ reported in Paper III when fixing γ = 13/3 in the search. Note that the lower x-axis scale is for A at f₀ = 1 yr⁻¹, whereas the upper x-axis is for A at f₀ = 10 yr⁻¹ (the normalization used in this paper). Since α = −2/3 for circular GW-driven binaries, there is a shift of 0.666 dex between the two.