I'll echo what El Pres and JohnS say. And you're correct, the D6 is definitely the most peppery. So much so that to me it's its own thing and not just a cut down D4. Blend and/or size may be why it's as different as it is to the D4 like El Pres stated.
To me the D5 though is exactly what you'd expect, somewhere in between. I've smoked a few of them this past year and have grown to really like them. I don't think they're peppery like the D6 but definitely richer than the D4. They're like a D4 with the volume turned up. I like all three a lot. Currently smoking more D4's though because they last longer and they're more mellow, which seems to be what I'm enjoying more of these days - and I have a lot more them on hand than the other two. Come winter I might be more in the mood for the D5 and D6 punch in the face.