We present an analysis of all BeppoSAX 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 ( described as a cut-off power law ) , 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 ( with variation of the photon index \Delta \Gamma \sim 0.2 ) .
The flux below 1 keV remains constant confirming an extended origin .
Its spectrum is reproduced by a combination of a thermal component ( with temperature kT = 0.15 keV ) 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 \sim 0.4 , with \Omega the solid angle covered by the reflecting medium ) , 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 .