I'm looking for a 4X5 scanner too (6x12cm to be precise) but I don't have the kind of expendable cash to buy a "real film scanner". With this personal limitation in mind I'm thinking of getting an Epson V700 and replace the glass with a sheet of aluminum painted flat black. Apertures through which the film can be seen by the sensor would be cut. This removes the glass from the image path thereby improving scan quality and greatly reducing dust problems.
Epson V700 - Custom Film Holder For Large Film
Any thoughts?