The way your r-4's are mounted on your tractor, they will give you the best traction going forward, in 2wd or 4wd. If you want more traction while backing, turn the front wheels around so the bars are pointing the other direction (forward if looking on top of the tire).
Sounds like either some additional weight in your ballast box, and/or fewer field stones will help you get the job started. Also, doing the job when the ground is drier will help as it sounds like you are having the most problem in wet, soft spots. You will gain some experience as you go along, and will find out what works and what doesn't. The outfit you have should work well for moving a lot of field stone. If you have a long way to travel, then a carrying platform on the 3ph that you could load field stone onto would give you rear weight as well as increase your efficiency per trip.
I sometimes work flat stone around (with the FEL) so I can prop it up in the air enough to back under it with a 3ph platform, saving on the bullwork as much as possible. Otherwise I roll the stones onto the platform. If the ground is dry, I will use the stone boat. But that doesn't give me any weight on the tractor wheels for traction.