Its been a 'while' now....but, from the limited info given,
Usually the VIS is f'cast as 10km (9999) or better, except where reduced by the associated phenomena - whatever it may be - e.g. dust, rain, showers etc. AS it occurs.
T'would be rare for the VIS to be 4000M for the WHOLE period of the FCST.
So, (c)....For the f'cast period of the Showers.
