  The CoRoT satellite has announced its fourth transiting planet (Aigrain et
al. 2008) with space photometry. We describe and analyse complementary
observations of this system performed to establish the planetary nature of the
transiting body and to estimate the fundamental parameters of the planet and
its parent star. We have analysed high precision radial-velocity data,
ground-based photometry, and high signal-to-noise ratio spectroscopy. The
parent star CoRoT-Exo-4 (2MASS 06484671-0040219) is a late F-type star of mass
of 1.16 Msun and radius of 1.17 Rsun. The planet has a circular orbit with a
period of 9.20205d. The planet radius is 1.19 Rjup and the mass is 0.72 Mjup.
It is a gas-giant planet with a ''normal'' internal structure of mainly H and
He. CoRoT-Exo-4b has the second longest period of the known transiting planets.
It is an important discovery since it occupies an empty area in the mass-period
diagram of transiting exoplanets.
