There is no simple answer as to what deck would replace the DR2d. The best answer is to look for another DR2d on ebay or over the web. They are out there. Hard to say what you need. Is the size of the deck an issue, what and how do you use the 4 channels? You don't give enough info and that is probably why you have not received an answer yet. There are pages and pages of topics here at TS on the many other 4 channel decks.
I don't particularly think the DR40 is the same as the Dr2d. It has XLR inputs and I think you have to use the internal mics for 4 channel, but I could be wrong about that, I don't own one myself. There are many 4 channel and greater decks discussed at great length in the "Recording Gear" section, but I know of none other either as small as the DR2d, or using 1/8 inch jacks for each pair of stereo channels, if those are your hot buttons.