I think Jerry is right. You can rather plan to have about 20% more traction when 4WD is engaged, if not more.
Is there a particular reason you wouldn't go with the R1s? I find the R4s to be a great compromise between Ags and Turfs as regards destruction of grass. I have yet to be really compromised because I had the industrials instead of the ags, but I don't know what all you'll be doing.
If you're going to be in muck a lot, you can plan on R4s filling up the lugs pretty quickly, and they don't drop it out as easily as R1s do in my opinion. But you probably know that even R1s can muck up enough to start losing their traction, and you'd be in the same boat.
John