We present the results of an extensive analysis of the ``ASCA AO2 observation of the Seyfert 1 galaxy NGC4051. The target exhibits broadband [0.5--10 keV] variability by a factor $sim 8$ on time scales $sim 10^4 s$, with a typical doubling time $sim 500 s$. The spectrum is characterized by a strong emission excess over the extrapolated power law at energies $E le 1 keV$. Absorption edges due to ionized oxygen species OVII and OVIII are detected together with an emission-like feature at $E sim 0.93 keV$. The OVII edge undergoes significant variability on a timescale as low as $sim 10^4 s$, whilst no contemporary variability of the OVIII feature is detected. Typical variability time scales place constraints on the location and the density of the absorbing matter. In the self-consistent hypothesis of a high energy ($E ge 2.3 keV$) power law reflected by an infinite plane-parallel cold slab, a photon index change ($Delta Gamma = 0.4$) has also been observed; a natural explanation can be found in the framework of non-thermal Comptonization models. The iron line is redshifted (centroid energy $E sim 6.1 keV$) and broad ($sigma > 0.2 keV$); multicomponent structure is suggestive of emission from a relativistic accretion disk; however if the disk is not ionized a contribution by a molecular torus or an iron overabundance by a factor $sim 1.5$ are required.
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