  The GJ 581 planetary system was already known to harbour three planets,
including two super-Earths planets which straddle its habitable zone. We report
here the detection of an additional planet -- GJ 581e -- with a minimum mass of
1.9 M_earth. With a period of 3.15 days, it is the innermost planet of the
system and has a ~5% transit probability. We also correct our previous
confusion of the orbital period of GJ 581d (the outermost planet) with a
one-year alias, thanks to an extended time span and many more measurements. The
revised period is 66.8 days, and locates the semi-major axis inside the
habitable zone of the low mass star. The dynamical stability of the 4-planet
system imposes an upper bound on the orbital plane inclination. The planets
cannot be more massive than approximately 1.6 times their minimum mass.
