Those are VERY different choices. Size, weight, price, full frame vs crop.
What are you going to be shooting? I loved my xpro1, but honestly, the AF wasn't that great. Last Christmas I upgraded to the XT-2. It's a huge upgrade, though you lose the rangefinder style and optical view finder.
I like a light set up, and like to shoot street photography when traveling, so the Fuji's are perfect for me. I have a toddler now, so the improved AF of the XT2 was huge (but I know it still is no match for the 4Ds)
If you like shooting zooms, sports/action, or have access to lots of nikon glass, then the 4Ds is probably the way to go.
What is the price of the 4Ds? For a mirrorless equivilent, isn't it more like the high end Sony's?I would think the xpro1 is a fraction of the price
I'm no pro or anything, but if you want I can post some samples from my Fuji's