EDIT: I'm about to put up the post with some of the recordings, pictures, and descriptions of what I captured on the U.S.S. Pampanito with the D50 on my site. There's a chance to win a copy of all of the sounds I recorded in the post.
I haven't used the NT4, but I have used the D-50. It's very clean for a handheld recorder. I've been meaning to get a post up on my website about the time Colin and I spent a little over an hour on a WWII sub recording switches, knobs and the like. All we had with us were handheld recorders, and I had the D50. I'll make sure to get that up tomorrow (with some of the sounds, of course) for everyone to check out.
The pres on the D50 are pretty good, but I haven't used that particular model of Fostex recorder to make a comparison.
It does have a screw hole for a standard consumer camera tripod on the back. So, it's easy to use with something like the Joby Gorillapod, or a larger camera tripod.
As with any handheld recorder, it's highly susceptible to handling noise and wind. something you should consider when making your decision. Then again, you'll want to consider that for the NT4 as well. I know people have been able to fit the NT4 into a blimp/zepplin. So that one affords you a bit more ease in terms of manipulation than the D50.