  Exoplanet systems with multiple transiting planets are natural laboratories
for testing planetary astrophysics. One such system is HD 191939 (TOI-1339), a
bright (V=9) and Sun-like (G9V) star, which TESS found to host three transiting
planets (b, c, and d). The planets have periods of 9, 29, and 38 days each with
similar sizes from 3 to 3.4 $R_{\oplus}$. To further characterize the system,
we measured the radial velocity (RV) of HD 191939 over 415 days with Keck/HIRES
and APF/Levy. We find that $M_b = 10.4 \pm 0.9 M_{\oplus}$ and $M_c = 7.2 \pm
1.4 M_{\oplus}$, which are low compared to most known planets of comparable
radii. The RVs yield only an upper-limit on $M_d$ (<5.8 $M_{\oplus}$ at
2$\sigma$). The RVs further reveal a fourth planet (e) with a minimum mass of
$0.34 \pm 0.01 M_{Jup}$ and an orbital period of 101.4 $\pm$ 0.4 days. Despite
its non-transiting geometry, secular interactions between planet e and the
inner transiting planets indicate that planet e is coplanar with the transiting
planets ($\Delta$i < 10$^{\circ}$). We identify a second high mass planet (f)
with 95% confidence intervals on mass between $2-11 \, M_{Jup}$ and period
between 1700-7200 days, based on a joint analysis of RVs and astrometry from
$Gaia$ and $Hipparcos$. As a bright star hosting multiple planets with
well-measured masses, HD 191939 presents many options for comparative planetary
astronomy including characterization with JWST.
