We study \gamma -ray observations of NGC 4151 by GRO /OSSE contemporaneous with X-ray observations by ROSAT and Ginga in 1991 June and with ASCA in 1993 May .
The spectra are well modeled by thermal Comptonization and a dual neutral absorber .
We also find , for the first time for NGC 4151 , a Compton-reflection spectral component in the Ginga /OSSE data .
When reflection is taken into account , the intrinsic X-ray energy spectral index is \alpha \sim 0.8 and the plasma temperature is \sim 60 keV for both observations , conditions which imply an optical depth of \sim 1 .
The X-ray spectral index is within the range , \alpha \simeq 0.95 \pm 0.15 , observed from other Seyfert 1s .
Also , the OSSE spectra of those and other observations of NGC 4151 are statistically undistinguishable from the average OSSE spectrum of radio-quiet Seyfert 1s .
Thus , NGC 4151 observed in 1991 and 1993 has the intrinsic X-ray/ \gamma -ray spectrum typical for Seyfert 1s , and the main property distinguishing it from other Seyfert 1s is a large absorbing column of \sim 10 ^ { 23 } cm ^ { -2 } .
We find no evidence for a strong , broad and redshifted , Fe K \alpha line component in the ASCA spectrum of 1993 May .
Also , the Compton-reflection component in the Ginga /OSSE spectrum is a few times too small to account for the strength of the broad/redshifted line reported elsewhere to be found in this and other ASCA spectra of NGC 4151 .

On the other hand , we confirm previous studies in that archival X-ray data do imply strong intrinsic X-ray variability and hardness of the intrinsic spectrum in low X-ray states .
An observed softening of the intrinsic X-ray spectrum with the increasing flux implies variability in \gamma -rays weaker than in X-rays , which agrees with the 100 keV flux changing only within a factor of 2 in archival OSSE and GRANAT /SIGMA observations .

The relative hardness of the intrinsic X-ray spectrum rules out the homogeneous hot corona/cold disk model for this source .
Instead , the hot plasma has to subtend a small solid angle as seen from the source of UV radiation .
If the hot plasma is purely thermal , it consists of electrons rather than e ^ { \pm } pairs .
On the other hand , the plasma can be pair-dominated if a small fraction of the power is nonthermal .