Color–magnitude diagram of H4 from our HST photometry. The HST system bands (F555W and F814W) have been transformed to Johnson- Cousins V and I magnitudes (see Sect. 2.2.1 for details). Only stars from the innermost 15″ around the cluster center are shown to minimize the impact of contaminating field stars. Dartmouth Isochrones (Dotter et al. 2008) of 7, 10, and 13 Gyr of age and a 10 Gyr ZAHB model are overplotted to the data. The ZAHB luminosity, however, is not sensitive to age in this regime and therefore the model is representative for all three ages compared here. Red stars indicate the position of the three likely H4 members in our sample and black circles are other targets with H4’s tidal radius. The isochrones give a best fit for ages around ~10 Gyr, but uncertainties in reddening and distance to the cluster do not allow a tight age constraint, although the chemical parameters of the cluster are now known.