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We study gamma-ray observations of NGC 4151 by GRO/OSSE contemporaneous withX-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 neutralabsorber. We also find, for the first time for NGC 4151, a Compton-reflectionspectral component in the Ginga/OSSE data. The OSSE spectra of those and otherobservations of NGC 4151 are statistically undistinguishable from the averageOSSE spectrum of radio-quiet Seyfert 1s. NGC 4151 observed in 1991 and 1993 hasthe intrinsic X-ray/gamma-ray spectrum typical for Seyfert 1s, and the mainproperty 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 andredshifted, Fe K$\alpha$ line component in the ASCA spectrum of 1993 May.  On the other hand, we confirm previous studies in that archival X-ray data doimply strong intrinsic X-ray variability and hardness of the intrinsic spectrumin low X-ray states. An observed softening of the intrinsic X-ray spectrum withthe 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 inarchival OSSE and Granat/SIGMA observations.  The relative hardness of the intrinsic X-ray spectrum rules out thehomogeneous hot corona/cold disk model for this source. Instead, the hot plasmahas to subtend a small solid angle as seen from the source of UV radiation. Ifthe hot plasma is purely thermal, it consists of electrons rather than $e^+e^-$pairs. On the other hand, the plasma can be pair-dominated if a small fractionof the power is nonthermal.Comment: 15 pages, 8 Postscript figures, MNRAS accepte

Broad-band -ray and X-ray spectra of NGC 4151 and their implications for physical processes and geometry