however required to confirm this.

Finally, we note that no a priori exclusion of active galactic nuclei (AGNs) has been performed in our mass-selected sample.

PROPERTIES OF VERY MASSIVE GALAXIES AT $3.0 \leq z < 4.0$ {#sec-prop}
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We use our mass-selected sample of 14 galaxies to determine the median and dispersion in observed and rest-frame properties of the most massive galaxies ($M_{\rm star} \gtrsim 2.5 \times 10^{11}$ M$_{\sun}$) at $3.0 \leq z < 4.0$. Table \[tab-2\] lists the median and 25th/75th percentiles of the distributions of observed $r$-band magnitude and $H-K$ color; the rest-frame $V$-band magnitude and $U-V$ color; and rest-frame UV slopes, parametrized by $F_{\rm \lambda} \propto \lambda^{\beta}$. Rest-frame UV slopes $\beta$ were determined from the best-fitting SEDs, following the @calzetti94 method of fitting to the 10 rest-frame UV bins defined by those authors.

Figure \[fig-histos\] shows the distributions of the observed $H-K$ color (top panel), rest-frame $U-V$ color (middle panel), and rest-frame UV slopes of the mass-selected sample (bottom panel). For comparison, we have also plotted 1) the distribution of rest-frame $U-V$ colors and UV slopes of a mass-selected sample of galaxies at $2<z<3$ with $M_{\rm star}>6\times10^{10}$ M$_{\sun}$ from @vandokkum06 (orange histogram); 2) the distribution of rest-frame 