GRI color–color plot for galaxies in our sample between 3.6 < z < 4.2. Filled and open circles represent respectively targets and serendipitous objects with optical counterparts. Black points show galaxies at all redshifts. The dashed box indicates the drop-out region in which galaxies at such redshifts are expected to lie. The cross in the bottom right part of the diagram shows the typical error bars.