I think you are making an incorrect generalization regarding "megapixels" in a scanned film slide. You might indeed be able to scan an 8x10 image at 4000dpi, but don't forget that film grain is going to be the limiting factor in terms of resolution when you push a scan that far, not your DPI. If you make an 8x10 drum scan at 4000dpi, you won't really be gaining resolution...you'll just be putting more pixels to each grain (especially for older photos like those from Ansel Adams...newer film like Velvia 50 4x5 slides actually have pretty good grain characteristics, although a 4000dpi scan is still going to be overkill). In general, when it comes to current 4x5 slid film landscape photographers, most don't scan much higher than 2800dpi (and at 8-bit rather than 16-bit color at that), and then only if they intend to print immensely large...300dpi to 900dpi are more common targets for 4x5 film users. A 2800dpi scan of a 4x5" or 1400dpi scan of 8x10" slide film is still 157mp.
One also has to determine the merits of the tradeoffs. A 1.3 gigapixel digital photo in 16-bit TIFF format is a 60 GIGABYTE image file. Not 60 megabytes, 60 GIGABYTES (and at 8-bit it would still be 30 gigs). In terms of a compressed TIFF file, the size on disk will be smaller, but if you wish to load that file into a tool like Photoshop, you are going to need an unholy amount of physical RAM to do so, on top of some immense swap disks as well. If you start working with layers, your working TIFF could quickly become 120, 240, 480 GIGS. At the moment, there really isn't a personal computer or even a high powered workstation on the planet that could handle that. You would have half a dozen bottlenecks, from bus bandwidth, to memory throughput to disk throughput (even assuming SATA-3 SSD), to video processing throughput, even CPU power. (Generally speaking, 4000dpi scans are for smaller formats, such as 35mm film scans, which result in digital images around 20-22mp...which is just middle-ground these days for DSLRs.)
On the flip side, an 8-bit TIFF of a 1400dpi 8x10 scan would be 3.8gigs. Still a scary-large image from an in-memory editing standpoint, bit less than 1/16th the size of a gigapixel image, and one rich with sharp detail at a pixel level, rather than detail where pixels are sub-grain size, having a rather soft and granular appearance at 100% (meaning your wasting a lot of computing power crunching pixels you don't actually need.) From a print standpoint, a 1400dpi scan would let you NATIVELY print over 90x70" @ 150ppi, so it is still way overkill in the general sense (if you are a specialist who prints 90x75, well you'd be set!) You could get away with a 900dpi scan at 8-bit, which would be a 65mp image, 1.6gb in-memory size, and capable of being printed at a native size of 60x40" @ 150ppi or 30x24" @ 300ppi. More than enough for pretty much ANY work one could think of.
The one benefit I can think of for an ungodly high DPI drum scan like 2800dpi would be for cropping. Out of a 627mp image, you could probably make a dozen separate "photos" from different areas of a single "shot", much like one of my favorite 4x5 landscape photographers demonstrates here:
http://www.widerange.org/gallery/resolution/
