Joe - I love the scanner, but must tell you that 90% of my use is with Vuescan, so my knowledge of the Nikonscan software is limited.
Dark scans? OK, lets assume you've got a reasonably well calibrated monitor. Which color space did you select for the scanner in the set up? Personally I prefer sRGB as I found it easier to get consistency across all software/devices. In some ways Adobe RGB is better, but more of a fiddle (I found) to get good print output.
BUT - and here's the key thing - if you are tempted down one of the "wider gamut" routes, you will get flatter and darker images.
Next - have you gone through the calibration routine in NikonScan - the bit where you have to match patches in lined boxes to get the gamma right. This I found very difficult to judge - it is important to use your normal working ambient lighting, and judge the patches a good 3 or 4 feet from the screen.
OK, assuming everything is set up right, are we talking scans darker than in preview, or dark in both preview & final? Ie. are you getting what you see on the screen, or is the final output significantly different.
Are you scanning K64? If so, have you selected the Kodachrome option for scanning (I missed the fact it was there for weeks!) makes a HUGE difference.
Which, if any, of the ICE/GEM/ROC are you using? While these can work wonders on poor quality originals, ROC in particularly can have odd effects on "normal" quality slides.
Finally, I don't think I've ever scanned anything in NikonScan which didn't require gamma adjustment - I use the Curves box a lot to bring out the midtones. I think what happens is that the scan is influenced by bright highlights (which we have perhaps more than the average in aviation photography) and sets the white point inappropriatly (ie. doesn't allow enough overhead). In the curves box you can choose your own white,black or mid-gray points. This alone might solve the problem, but if not, some manual adjustment of the curve should fix things.
BTW - your thinking on purchasing the 4000ED was much the same as mine. The IV is a fine scanner, but having compared the output of both, the 4000ED was so clearly superior I knew I would never be happy with the IV!
Colin K. Work, Pixstel