Are you asking how many pixels a digital camera would have to have in order to equal 4x5 film in resolution?

There is no exact answer to this question because randomly positioned film grain and periodic pixels are different. Back-of-the-envelope calculations suggest anything from several tens of mega-pixels to around 200 megapixels. If you are thinking of color and want 48 bits per pixel (16 bits per color), then you need to multiply the pixel size by 6 bytes per pixel to obtain the file size.

Some estimates based on scanning film are given by R. N. Clark: http://www.clarkvision.com/imagedetail/index.html and http://www.clarkvision.com/imagedetail/film.vs.digital.1.html