  The microquasar GX 339-4 experienced a new outburst in 2010: it was observed
simultaneously at various wavelengths from radio up to soft gamma-rays. We
focused on observations that are quasi-simultaneous with those made with the
INTEGRAL and RXTE satellites: these were collected in 2010 March-April during
our INTEGRAL Target of Opportunity program, and during some of the other
INTEGRAL observing programs with GX 339-4 in the field-of-view. X-ray
transients are extreme systems that often harbour a black hole, and are known
to emit throughout the whole electromagnetic spectrum when in outburst. The
goals of our program are to understand the evolution of the physical processes
close to the black hole and to study the connections between the accretion and
ejection. We analysed radio, NIR, optical, UV, X-ray and soft gamma-ray
observations. We studied the source evolution in detail by producing light
curves, hardness-intensity diagrams and spectra. We fitted the broadband data
with phenomenological, then physical, models to study the emission coming from
the distinct components. Based on the energy spectra, the source evolved from
the canonical hard state to the canonical soft state. The source showed X-ray
spectral variations that were correlated with changes in radio, NIR and optical
emission. The bolometric flux increased from 0.8 to 2.9*10^{-8} erg cm^{-2}
s^{-1} while the relative flux and contribution of the hot medium globally
decreased. Reprocessing in the disc was likely to be strong at the end of our
observations. The source showed a behaviour similar to that of previous
outbursts, with some small deviations in the hard X-rays parameters' evolution.
The radio, NIR and optical emission from jets was detected, and seen to fade as
the source softened. The results are discussed within the context of disc and
jet models.
