  We present results from a Keck/HIRES radial velocity campaign to study four
sub-Saturn-sized planets, K2-27b, K2-32b, K2-39b, and K2-108b, with the goal of
understanding their masses, orbits, and heavy element enrichment. The planets
have similar sizes $(R_P = 4.5-5.5~R_E)$, but have dissimilar masses $(M_P =
16-60~M_E)$, implying a diversity in their core and envelope masses. K2-32b is
the least massive $(M_P = 16.5 \pm 2.7~M_E)$ and orbits in close proximity to
two sub-Neptunes near a 3:2:1 period commensurability. K2-27b and K2-39b are
significantly more massive at $M_P = 30.9 \pm 4.6~M_E$ and $M_P = 39.8 \pm
4.4~M_E$, respectively, and show no signs of additional planets. K2-108b is the
most massive at $M_P = 59.4 \pm 4.4~M_E$, implying a large reservoir of heavy
elements of about $\approx50~M_E$. Sub-Saturns as a population have a large
diversity in planet mass at a given size. They exhibit remarkably little
correlation between mass and size; sub-Saturns range from $\approx 6-60~M_E$,
regardless of size. We find a strong correlation between planet mass and host
star metallicity, suggesting that metal-rich disks form more massive planet
cores. The most massive sub-Saturns tend to lack detected companions and have
moderately eccentric orbits, perhaps as a result of a previous epoch of
dynamical instability. Finally, we observe only a weak correlation between the
planet envelope fraction and present-day equilibrium temperature, suggesting
that photo-evaporation does not play a dominant role in determining the amount
of gas sub-Saturns accrete from their protoplanetary disks.
