Not an expert either, but my understanding: bit depth is a measure of the amount of color shades/levels of variation that are captured for each color in a file. So, a 4 bit scan basically has 25% of the shade levels compared to a 16-bit file. That means that when printing, there won't be as subtle a change in the the image of, say, a green field -- instead of the file being able to use 200 shades of green in a 16 bit file, a 4-bit file might only have 50 shades (the numbers aren't accurate, I'm just using them to show the difference). So, if you have a photo with rich colors and a lot of different variations in color, etc. You are losing out if you go 4-bit. It's a similar decision to choose MF vs. 35 mm....
So, if you scan 16-bit JPEGs, you have more information to work with. When blowing up a file, more info is better (usually) -- more data to work with. If you were talking snapshots, I would think 4-bit would be fine. But if you're shooting MF, I assume you're doing so because you love the quality of the detail a big negative gives you, right? Scanning 4-bit to me is like driving a Ferrari but using the cheapest/lowest octane gas you can find -- it won't give you the experience you want - why buy the Ferrari if you're not going to use it to its fullest potential? (Again, this is only if you are into the MF experience and want to blow up your pics and maintain the quality of the image).
You have to make a similar decision in file type -- JPEG vs. TIF or RAW. RAW/TIF saves much more information; JPEG extrapolates information and saves what it thinks is important, thereby making a smaller file. When scanning, y ou can usuallly get either JPEG or TIF (in the shops). Again, if you scan a file as a JPEG, the computer decides what info is kept and what is thrown away. JPEG does a great job, but if you are someone who likes to manipulate your files afterwards, then you're working with less info. TIF files capture a lot more info (and of course are then a lot bigger) and are more expensive. But again, if you're using MF to get the rich big-file experience, you might want to think about doing TIFs. At least for your favorite images.
What I've beend doing lately is scanning 4-bit JPEGS of every roll of MF film and seeing which of the negatives have a lot of potential. Then I'll go back and scan those as 16-bit TIF files so i can play with them. That's the beauty of scans -- you can repeat. So you can do your first scans less expensive, and they work great for Flickr, etc. If there are a few you love and you want to print large prints, you can always go back and scan again selectively to save $$, and then use that file to get the size/quality you want.
Oh, and last (I'm sure you know this), you need to look at the DPI of the scan as well if you are just interested in blowing up a scanned JPEG file and printing large without manipulating. So it's 3 things to consider (Bit-depth, file type and DPI) when you're thinking about scanning. And the decisions depend on what exactly you want to do with the image (and of course how much you're willing to pay
EFD
