You guys probably know this, but just in case, the DR-2D doesn't have four inputs like the R4. You are limited to a stereo line input, and the onboard mics if you want to do a four track. You can't do a stereo board feed and a stereo external mic feed at the same time.
The DR-40 is the same, but the interface was a good bit better to work with in my experience.
If using the onboard mics as your open air mics isn't a killer, I'd recommend the DR-40 over the DR-2D despite the higher cost ($200 vs $125). The DR-40 is bigger than the DR-2D, but I suspect also significantly smaller than the R4.
Milo