4x6 inches??? You have to be cropping a huge amount not to have resolution for that. At 300 dpi, that's 1200 x 1800 or about 2mp. A sixth of the frame; call it a quarter in each direction. And that's native resolution, of a 12mp camera. If you're having to crop 85% of the frame, you're shooting with the wrong lenses. Even an 8x10 at 300 dpi is still about 7mp. Even that is a significant crop from 12mp, let alone from 24mp. And we're not even discussing a controlled upres.
I'm sure that some will scoff at my ridiculously low standards (in their opinion), but really, I have some 16x20s that were made from D2h files. They are not the killer shots from a D800e with a tripod-mounted 60/f2.8 AFS Micro, but they stand up to viewing. I have some prints in the rec room that are somewhere around 17x22; they were made from mild crops from 12mp cameras. No, I suppose they are not Ansel Adams from an 8x10 view camera, but most folks are pretty impressed at the detail. I just checked those two files in LR - they are 8.8mp and 9.1mp respectively, after cropping. It's hard to believe that you can't get what you need from a D3.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!