This mimics what I was going to say about the tires.The lack of traction seems to be a common complaint with the r4 tires. A coworker had them on his Kubota and tried using it to clear his driveway, said it kept slipping and sliding.
I know there is a weight difference between the old and new machine, but I think it's the tires more than anything. The R4 wont rip things to hell, but that being said, they aren't biting like the AG tires you had on the old tractor would. I would say get used to using the diff lock.