z_850LP − 4.5 μm observed colour histogram at z ~ 4. In red, we have best fit objects with τ ≥ 100 Myr, and in blue, best fit objects with τ = 10 Myr for the three models with decreasing SF. The dashed blue, red, and black lines show respectively a median colour for objects that are best fit with τ = 10 Myr, with τ ≥ 100 Myr and for the whole sample. For the two subsamples, KS test for the three models gives p = 7.8 × 10^-8, p = 2.6 × 10^-17, and p = 1.3 × 10^-16, respectively, for DEC, DEC+NEB+Lyα, and DEC+NEB models, showing that the two subsamples are not drawn from the same population.