The D4 is more balanced and neutral sounding than the D2 Boa (haven't heard the D2+). And the D4 DAC seems to be about as detailed as the Pico's (slightly drier and less dynamic, but very close). That right there is a plus.
Add to that the ability to drive an HD600 or HE-5 when on 9v battery power better than the D2 can with it's 5v supply, and I would pay the extra for the D4. With the UM3X and W3 the difference in power may not be as much of an issue, but I'm sure someday you are going to want to try full size headphones and the difference is noticeable. And, the improved frequency balance will be an improvement with the IEM regardless of power.