I split the baby in the middle I have an Epson 4990 and it does a very credible job on 4x5 chromes giving me more detail than I get from my 1Ds2. If I need a significantly higher quality file -- which has not happened yet -- I figure I can send it out and have it drum-scanned.
Or you could move up to an 8x10" format. Doesn't the Epson 4990 scan 8x10" film?
I've never used a larger format than 6x9cm, so I might well not know what I'm talking about, but from the little I do know about film and lens MTF and resolution, the maths implies that the best digital MF cameras have a long way to go before they could match 8x10 film.
For example, there's a lens designed for the Gigapxl project that apparently (?)delivers at f22 30lp/mm in the centre at 50%MTF, falling off towards the edges of course. The lens is designed for a 9x18" format. When used with 8x10" format, the fall off would be less significant. I'd estimate about 30 lp/mm at 30% MTF at the edges.
But lets use a conservative figure of 20 lp/mm which would be above 50% MTF over most of the film area. The MTF curves for color film would suggest that some types of film can record 20 lp/mm without any noticeable loss in contrast at all. Ie. their MTF response up to 20 lp/mm is 100%. In fact, it's sometimes above 100%, implying a certain degree of natural sharpening.
Converting to mm we get 8x10 = 200x250mm. At 20 lp/mm we get a total picture resolution of 4,000x5,000 line pairs. At best it takes 2 Foveon type pixels to record 1 line pair or 3 Bayer type pixels, so by my calculation a digital camera that could equal the resolution of film based 8x10" format would have to be an 80MP Foveon type sensor or a 160-200MP Bayer type sensor.
Whilst grain (or clumps of grain) is (are) a major problem with film, often degrading fine detail, the larger the format the smaller the problem. For example, an average size clump of grain on 35mm Provia F which might be quite noticeable, is effectively less than 1/50th the size on a piece of 8x10" film, as seen on the same size print. (Or perhaps I should say, as not
seen on the same size print.)