R44 is pretty much scaled down version of the R4 without the harddrive. It would be a direct replacement for your R4 if paired with a 32GB card. R44 is a straight-forward 4 channel machine in the traditional sense. I use an Oade Concert Modded version, skip the external preamp and power it with a DVD battery. The recorder and battery together are smaller than the R4 alone. Great full featured machine that isn't too large.
The DR2d strong points are mostly small size and cost and not needing an external battery. The DR2d is far smaller still (its a pocketable hand-held) but is really a totally different animal- 3.5mm inputs only, no phantom, more difficult to adjust gain, and not really designed as a 4 channel machine with that as a primary function. It's more of an R09 that can also record a second pair with some gain adjustment limitations. I've replaced two R09s with DR2ds. Two sync'd pairs of channels in one pocket.
SD744 is 4 channel, different dimensions but about the size of the R44, and is built to Sound Devices quality standards at a significantly higher price- one limitation compared to the R44 is that it has only two built-in preamps.
Most if not all of these machines which have XLR inputs use XLR - 1/4"TRS combo jacks.