Seismic HRD of M4. Here we show stars in the log L − T_eff plane for which masses are available. The colors indicate stellar mass. The gap in the HB between RRc and rHB stars is where RRab stars reside and for which no mass estimate is available. The solid line is a MIST isochrone with an age of 12.2 Gyr and an [Fe/H] index of –0.9. Two BaSTI ZAHB tracks ([α/Fe] = 0.4; [Fe/H] = –0.9 (lower) and [Fe/H] = –1.2 (upper) are also shown.