  (Abreviated) Continued radial velocity monitoring of the nearby M4V red dwarf
star GJ~876 with Keck/HIRES has revealed the presence of a Uranus-mass fourth
planetary companion in the system. The new planet has a mean period of
$P_e=126.6$ days (over the 12.6-year baseline of the radial velocity
observations), and a minimum mass of $m_e\sin{i_e}=12.9\pm 1.7\,M_{\oplus}$.
Self-consistent, N-body fits to the radial velocity data set show that the
four-planet system has an invariable plane with an inclination relative to the
plane of the sky of $i=59.5^{\circ}$. The fit is not significantly improved by
the introduction of a mutual inclination between the planets ``b'' and ``c,''
but the new data do confirm a non-zero eccentricity, $e_d=0.207\pm0.055$ for
the innermost planet, ``d.'' In our best-fit coplanar model, the mass of the
new component is $m_e=14.6\pm1.7\,M_{\oplus}$. Our best-fitting model places
the new planet in a 3-body resonance with the previously known giant planets
(which have mean periods of $P_c=30.4$ and $P_b=61.1$ days). The critical
argument, $\varphi_{\rm Laplace}=\lambda_c-3\lambda_b+2\lambda_e$, for the
Laplace resonance librates with an amplitude of $\Delta\varphi_{\rm
Laplace}=40\pm13^{\circ}$ about $\varphi_{\rm Laplace}=0^{\circ}$. Numerical
integration indicates that the four-planet system is stable for at least a
billion years (at least for the coplanar cases). This resonant configuration of
three giant planets orbiting an M-dwarf primary differs from the well-known
Laplace configuration of the three inner Galilean satellites of Jupiter, which
are executing very small librations about $\varphi_{\rm Laplace}=180^{\circ}$,
and which never experience triple conjunctions. The GJ~876 system, by contrast,
comes close to a triple conjunction between the outer three planets once per
every orbit of the outer planet, ``e.''
