I present the evolution of the stellar mass function (SMF) of galaxies
from z=4.0 to z=1.3 measured from a sample constructed from the deep
NIR MUSYC, the FIRES, and the GOODS-CDFS surveys, all having very
high-quality optical to mid-infrared data. I provide, for the first
time, a comprehensive analysis of random and systematic uncertainties
affecting the derived SMFs. I find that the mass density evolves by a
factor of ~17 since z=4.0, and a factor of ~4 from z=1.3. The
evolution appears to be mostly driven by a change in the normalization
Phi*, but I also find evidence for evolution in the shape of the SMF,
with the low-mass end evolving more rapidly than the high-mass
end. Taking these results at face value, I find that they are in
conflict with the predictions of semi-analytic models of galaxy
formation. I will conclude by presenting preliminary results on the
evolution of the massive end of the SMF from z=4 from the NEWFIRM
Medium-Band Survey, a large NOAO/Yale program which uses medium
band-width filters in the near-IR to obtained well-sampled spectral
energy distributions and high quality photometric redshifts over 1
square degree, returning redshifts accurate to 0.02 in Delta z / (1+z)
over the redshift range 1.5<z<3.5.