  NGC1052-DF4 was found to be the second "galaxy lacking dark matter" in the
NGC1052 group, based on its velocity dispersion of $\sigma_{\rm
gc}=4.2^{+4.4}_{-2.2}$ km/s as measured from the radial velocities of seven of
its globular clusters. Here we verify this result by measuring the stellar
velocity dispersion of the galaxy. We observed the diffuse stellar light in
NGC1052-DF4 with the Keck Cosmic Web Imager (KCWI) in its highest resolution
mode, with $\sigma_{\mathrm{instr}}\approx 7$ km/s. With a total science + sky
exposure time of 34hrs, the resulting spectrum is exceptional both in its
spectral resolution and its S/N ratio of 23\r{A}$^{-1}$. We find a stellar
velocity dispersion of $\sigma_{\rm stars} = 8.0^{+2.3}_{-1.9}$ km/s,
consistent with the previous measurement from the globular clusters. Combining
both measurements gives a fiducial dispersion of $\sigma_{\rm f} =
6.3_{-1.6}^{+2.5}$ km/s. The implied dynamical mass within the half-light
radius is $8_{-4}^{+6} \times 10^7 M_{\odot}$. The expected velocity dispersion
of NGC1052-DF4 from the stellar mass alone is $7 \pm 1$ km/s, and for an NFW
halo that follows the stellar mass -- halo mass relation and the halo mass --
concentration relation, the expectation is $\sim 30$ km/s. The low velocity
dispersion rules out a normal NFW dark matter halo, and we confirm that
NGC1052-DF4 is one of at least two galaxies in the NGC1052 group that have an
anomalously low dark matter content. While any viable model for their formation
should explain the properties of both galaxies, we note that NGC1052-DF4 now
poses the largest challenge as it has the most stringent constraints on its
dynamical mass.
