  We present an analysis of all SAX observations of NGC 4151. This source was
observed 5 times from 1996 to 2001 with durations ranging from a day to four
days. The intrinsic continuum, is absorbed at low energies by a complex system:
a cold patchy absorber plus a warm uniform screen photoionized by the central
continuum. We find that this dual absorber is the main driver of the observed
variability, up to a factor of eight, at 3 keV. In particular the covering
fraction of the cold absorber changes on time scales of the order of a day,
supporting its association with the Broad Line Region. The column density of
the warm gas varies on a longer time scale (months to year). Some of the small
amplitude spectral variability above 10 keV can be explained with an intrinsic
variation (\Delta\Gamma~0.2).The flux below 1 keV remains constant confirming
an extended origin. Its spectrum is reproduced by a combination of a thermal
component and a power law with the same slope as the intrinsic continuum but
with an intensity a few per cent. A Compton reflection component is
significantly detected in 1996 (averaged value of \Omega/2\pi ~0.4), with
intensity decreasing on time scale of year, and it desappears in 2000 and 2001.
The long time scale of variations argues for an association with an optically
thick torus at a distance of few light years. An iron line was detected in all
spectra. Its energy is consistent with fluorescence by cold iron. We find that
the line is variable. Its behaviour is reproduced by a variable component
proportional to the level of the reflection flux plus a constant component. The
flux of the latter is consistent with the extended line emission observed by
Chandra. We conclude that the first component is likely arising from the torus
and the second is produced in the extended Narrow Line Region.
