  The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances
down to 27.9 Rs, allowing solar wind turbulence and acceleration mechanisms to
be studied in situ closer to the Sun than previously possible. The turbulence
properties were found to be significantly different in the inbound and outbound
portions of PSP's fourth solar encounter, likely due to the proximity to the
heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the
streamer belt wind, the turbulence was found to have lower amplitudes, higher
magnetic compressibility, a steeper magnetic field spectrum (with spectral
index close to -5/3 rather than -3/2), a lower Alfv\'enicity, and a "1/f" break
at much lower frequencies. These are also features of slow wind at 1 au,
suggesting the near-Sun streamer belt wind to be the prototypical slow solar
wind. The transition in properties occurs at a predicted angular distance of
~4{\deg} from the HCS, suggesting ~8{\deg} as the full-width of the streamer
belt wind at these distances. While the majority of the Alfv\'enic turbulence
energy fluxes measured by PSP are consistent with those required for
reflection-driven turbulence models of solar wind acceleration, the fluxes in
the streamer belt are significantly lower than the model predictions,
suggesting that additional mechanisms are necessary to explain the acceleration
of the streamer belt solar wind.
