Well I wouldn't - unless you want to spread 4 inches of topsoil all over it and then lay onto that, but the results won't be great - bought in topsoil is usually heavy, clay motorway spoil, complete with weed seeds, unless you pay absolutely top dollar for graded topsoil. Either scrape up to a friable top inch or two the surface of the existing lawn and apply seed, or remove what's there, turn the soil over, level off, walk on it to get rid of soft spots, level again and lay new turf.