Canon makes the 9950F which is in the same class.  Also, another difference
between the 4990 and i900 is that the 4990 has ICE for film.  If I am not
mistaken, the i900 has ICE that only works on prints.

Doug

There are lots of reviews and sample scans
from the Epson 4870.  The consensus is that
this scanner's "real" resolution is roughly
half its advertised resolution.  IOW, it may
well be enough for your MF scans, but
(IMO) not for 35mm.

My one concern with the Microtek would be
banding, with which I've had a good deal
of personal experience, on every Microtek
scanner (or Microtek-designed scanner)
that I've owned.  (I use a Microtek 2500
for scanning 4x5 film.)

Take a look also at the Canon 9950, there
are reviews and sample scans.  Looks to me
roughly on par with the Epson 4870.

If you really care about the scans, I'd
suggest springing for a good used Nikon
LS-8000, which is in another league altogether.

rafe b.
http://www.terrapinphoto.com

What are your price limits (none of us wants to spend bazillion dollars but
more specificity than that would be helpful, as would information about what
your likely maximum print sizes will be with the various formats and what
you plan to do with the prints).

FWIW, I don't think you're going to find a single scanner that allows you to
make excellet prints  in the 11x14 and up range with everything from 35mm to
4x5 without spending the bazillion dollars you don't want to spend. A 4x5
film scanner is pretty much out of the question unless you're spending
around $5,000 or more (or unless prices of 4x5 film scanners have come way
down since I last looked). So you're probably looking at a flat bed of some
sort for 4x5. The Epson scanners that preceded the 4990 did quite well for
4x5 with prints in the 11x14 - 16x20 range and o.k. for medium format if the
prints sizes were kept around 8x10 to 11x14, especially with 6x7 and 6x9.
I'd assume the 4990 will do somewhat better with those film formats and
those sizes. However, from all I've read none of the Epson flat beds have
been any good for 35mm scans except for tiny prints or web posting. I doubt
that the 4990 will be much better with 35mm since its realistic resolution
is probably about 2500, which translates to roughly an 8x10 print at best
from 35mm at 300dpi.

So my guess, without more detailed information of the kind outlined above,
is that you need to think in terms of two scanners, a film scanner for 35mm
and a flatbed of the 4990 variety for 4x5 and hope it works well for your
various medium formats.  Of course for web posting almost anything that will
get the image into your computer will be o.k.