dilemma would be if one could sync the 2 digital audio together,since it might be a probability that the sd box ad converter has a delay/speed filter etc different from an outboard box.
This all depends on clocking. If you're going with 2 different digital inputs, the devices all need to be locked to the same master clock. If that can't be done (example - V3 and MiniMe can't be slaved, to my knowledge), the recorder will have to reclock/resample on the fly to prevent sync issues.
If you can do one analog + one digital, it seems like the SD box could slave to the outboard A/D and record all 4 tracks in sync. This is what my ULN-2 does (and it also has the SRC on the fly, should I desire that).