  An analysis of solar neutrino data from the fourth phase of
Super-Kamiokande~(SK-IV) from October 2008 to May 2018 is performed and the
results are presented. The observation time of the data set of SK-IV
corresponds to $2970$~days and the total live time for all four phases is
$5805$~days. For more precise solar neutrino measurements, several improvements
are applied in this analysis: lowering the data acquisition threshold in May
2015, further reduction of the spallation background using neutron clustering
events, precise energy reconstruction considering the time variation of the PMT
gain. The observed number of solar neutrino events in $3.49$--$19.49$ MeV
electron kinetic energy region during SK-IV is
$65,443^{+390}_{-388}\,(\mathrm{stat.})\pm 925\,(\mathrm{syst.})$ events.
Corresponding $\mathrm{^{8}B}$ solar neutrino flux is $(2.314 \pm 0.014\,
\rm{(stat.)} \pm 0.040 \, \rm{(syst.)}) \times
10^{6}~\mathrm{cm^{-2}\,s^{-1}}$, assuming a pure electron-neutrino flavor
component without neutrino oscillations. The flux combined with all SK phases
up to SK-IV is $(2.336 \pm 0.011\, \rm{(stat.)} \pm 0.043 \, \rm{(syst.)})
\times 10^{6}~\mathrm{cm^{-2}\,s^{-1}}$. Based on the neutrino oscillation
analysis from all solar experiments, including the SK $5805$~days data set, the
best-fit neutrino oscillation parameters are $\rm{sin^{2} \theta_{12,\,solar}}
= 0.306 \pm 0.013 $ and $\Delta m^{2}_{21,\,\mathrm{solar}} = (6.10^{+
0.95}_{-0.81}) \times 10^{-5}~\rm{eV}^{2}$, with a deviation of about
1.5$\sigma$ from the $\Delta m^{2}_{21}$ parameter obtained by KamLAND. The
best-fit neutrino oscillation parameters obtained from all solar experiments
and KamLAND are $\sin^{2} \theta_{12,\,\mathrm{global}} = 0.307 \pm 0.012 $ and
$\Delta m^{2}_{21,\,\mathrm{global}} = (7.50^{+ 0.19}_{-0.18}) \times
10^{-5}~\rm{eV}^{2}$.
