  In the next decades, it is necessary to forge new late-universe cosmological
probes to precisely constrain the Hubble constant and the equation of state of
dark energy simultaneously. In this work, we show that the four typical
late-universe cosmological probes, the 21 cm intensity mapping (IM), fast radio
burst (FRB), gravitational wave (GW) standard siren, and strong gravitational
lensing (SGL), are expected to be forged into useful tools in solving the
Hubble tension and exploring dark energy. We propose that the synergy of them
is rather important in cosmology. We simulate the 21 cm IM, FRB, GW, and SGL
data based on the hypothetical observations of the Hydrogen Intensity and
Real-time Analysis eXperiment (HIRAX), the Square Kilometre Array (SKA), the
Einstein Telescope (ET), and the Large Synoptic Survey Telescope (LSST),
respectively. We find that the four probes show obviously different parameter
degeneracy orientations in cosmological constraints, so any combination of them
can break the parameter degeneracies and thus significantly improve the
constraint precision. The joint 21 cm IM+FRB+GW+SGL data can provide the
constraint errors of $\sigma(\Omega_{\rm m})=0.0022$ and $\sigma(H_0)=0.16\ \rm
km\ s^{-1}\ Mpc^{-1}$ in the $\Lambda$CDM model, which meet the standard of
precision cosmology, i.e., the constraint precision of parameters is better
than 1%. In addition, the joint data give $\sigma(w)=0.020$ in the $w$CDM
model, and $\sigma(w_0)=0.066$ and $\sigma(w_a)=0.25$ in the $w_0w_a$CDM model,
which are all better than the constraints obtained by the CMB+BAO+SN data. We
show that the synergy between the four late-universe cosmological probes has
magnificent prospects.
