So it seems that every scanner is starting to claim values like 3.9, 4.0, 4.1 and some even 4.2, even though the theoretical 4.0 is the highest value possible. I have not gotten a good explanation of this from anyone. In the actual scan these differences will mainly surface in shadow detail. The best way to determine this before buying a scanner is take a transparency that has a very wide latitude of shadow to highlights and compare the scanner you want to buy Vs. a good drum scanner. If the difference is marginal but the price difference is substantial, then you have the answer you need to make an intelligent choice based on you needs.
Bit depth is the number of colors a scanner can capture.
And although good drum scanners have 16 bits per channel input for a total of 48 bits, this is not that useful if you bring the file into PhotoShop where it immediately gets compressed to 8 bit channels. However, if the file was intended to go to an output that would take advantage of this extra bit depth, such as a film recorder, then this would probably make a good bit of difference. I have not tried this myself. From most articles I read it still pays to capture the original file with greater bit depth even if it will be compressed in PhotoShop. Once again, if one thinks this is an issue when buying a scanner, try both and compare the outputs of the prints.