We report on the results of new Ginga observations of the archetypal Seyfert galaxy, NGC 4151, performed over an 18-month period between early 1990 and mid-1991. The present programme complements earlier extensive monitoring of this source in the medium-energy X-ray band by both EXOSAT and Ginga to give a combined data set of some 36 separate observations made over a period of eight years. The most interesting aspect of the new observations is that they bring to light a remarkable transient change in the low-energy extinction and soft X-ray excess apparent in the X-ray spectrum of NGC 4151. The spectrum became systematically softer below ~4 keV from mid-1988, with the spectral softness peaking in mid-1990 and reverting to its former state by late 1990. Interpreting this in terms of the partial covering model, the covered fraction of the source, which normally takes a value of 80-90 per cent, underwent a sharp decline to ~ 20 per cent over a two-year period and then returned to the canonical level over the following six months. This observation would seem to rule out a cloud model for the X-ray absorbing medium in NGC 4151 and favour the presence of a normally stable `bulk' structure such as an edge-on accretion disc. A transitory change of azimuthal structure in the disc could then account for the apparent dip in the covered fraction. If the central X-ray source radiates isotropically then the medium producing the bulk of the iron-line emission must deviate from a spherically symmetric distribution in such a way that its column density in most directions is at least 6 x 10^22^ cm^-2^ but <~ 10^22^ cm^-2^ in our line of sight. However, a more reasonable model may be one in which there is substantial anisotropic X-ray illumination of broad-line clouds out of our direct line of sight, in which case the true X-ray luminosity of NGC 4151 may be much higher than is generally assumed.