  We present a comprehensive spectral analysis of all INTEGRAL data obtained so
far for the X-ray--bright Seyfert galaxy NGC 4151. We also use all
contemporaneous data from RXTE, XMM, Swift and Suzaku. We find a linear
correlation between the medium and hard-energy X-ray fluxes measured by
INTEGRAL, which indicates an almost constant spectral index over six years. The
majority of INTEGRAL observations were made when the source was either at a
very bright or very dim hard--X-ray state. We find that thermal Comptonization
models applied to the bright state yields the plasma temperature of 50--70 keV
and its optical depth of 1.3--2.6, depending on the assumed source geometry.
For the dim state, these parameters are in the ranges of 180--230 keV and
0.3--0.7, respectively. The Compton parameter is y = 1 for all the spectra,
indicating a stable geometry. Using this result, we can determine the
reflection effective solid angles associated with the close and distant
reprocessing media as = 0.3 x 2pi and 0.2 x 2pi, respectively. The plasma
energy balance, the weak disc reflection and a comparison of the UV fluxes
illuminating the plasma to the observed ones are all consistent with an inner
hot accretion surrounded by an outer cold disc. The disc truncation radius can
be determined from an approximate equipartition between the observed UV and
X-ray emission, and from the fitted disc blackbody model, as 15 gravitational
radii. Alternatively, our results can be explained by a mildly relativistic
coronal outflow.
