Extragalactic foreground power spectra for S4-SAT (coloured dashed lines) and S4-LAT (coloured continuous lines) at 145 GHz. The three continuous black lines are the primordial CMB B-mode power spectrum for r = 0.1, 0.01, and 0.001 from top to bottom. The dash-three-dots line is the lensing B-mode. As the two experiments are at the same frequency, the two curves are confounded.