  We present the census of massive (log(M$_{*}$/M$_{\odot}$)$\geq 11$) galaxies
at $3<z<6$ identified over the COSMOS/UltraVISTA Ultra-Deep field stripes:
consisting of $\approx100$ and $\approx20$ high-confidence candidates at
$3<z<4$ and $4<z<6$, respectively. The $3<z<4$ population is comprised of
post-starburst, UV star-forming and dusty-star forming galaxies in roughly
equal fractions, while UV-star-forming galaxies dominate at $4<z<6$ . We
account for various sources of biases in SED modelling, finding that the
treatment of emission line contamination is essential for understanding the
number densities and mass growth histories of massive galaxies at $z>3$. The
significant increase in observed number densities at $z\sim4$ ($>\times$ 5 in
$\lesssim600$ Myrs) implies that this is the epoch at which
log(M$_{*}$/M$_{\odot}$)$\geq 11$ galaxies emerge in significant numbers, with
stellar ages ($\approx500-900$ Myrs) indicating rapid formation epochs as early
as $z\sim7$. Leveraging ancillary multi-wavelength datasets, we perform
panchromatic SED modelling to constrain the total star-formation activity of
the sample. The star-formation activity of the sample is generally consistent
with being on the star-formation main sequence at the considered redshifts,
with $\approx15-25\%$ of the population showing evidence of suppressed
star-formation rates, indicating that quenching mechanisms are already at play
by $z\sim4$. We stack available HST imaging, confirming their compact nature
($r_{e}\lesssim2.2$ kpc), consistent with expected sizes of high-$z$
star-forming galaxies. Finally, we discuss how our results are in-line with the
early formation epochs and short formation timescales inferred from the fossil
records of the most massive galaxies in the Universe.
