I can't remember trying with DPP4,. but strangely I find that a LOT of Graphic application software offers terrible, limited, or even non existent support for dual monitors.
Photoshop was fully Dual monitor supporting as far back as 2003 -ish, with dockable toolbars and or image windows that could be placed on any monitor, with the layout savable.
10 years later I was dumping RAW converters based solely on their inability to allow me to use Dual Monitors. In one case, and excellent RAW converter (Bibble) went from very good multi monitor support to very poor multi monitor support with a supposed "upgrade" !!
So may right now claim "dual" or "multi" monitor support when all you can really get is a set of thumbs on the 2nd monitor.
I ONLY consider a graphics app to support Dual Monitors IF I can have one monitor displaying nothing but the image, and the other monitor used for toolbars, thumbs, file structure etc.. If you can't let me view the image alone, then your not giving me any advantage to multi monitors.
Capture 1 and Photoshop are the two that I know will allow me to do this.