I ordered a set of 1" spacers. Here is the story I got from Carver Equipment.
There was an intial problem with the R4s being too close to the fenders when these tractors first came out. Kubota rectified the problem with wheel spacers and permanently fixed it by modifying the rims a little to get some more space between the tire and fender.
I must have got an older set of rims as it is impossible to slide my hand between the sidewall and fender. The 1" spacers will help a little. Carver was one of 2 online dealers of about 6 that I called that knew of the spacers. The others said the kit did not exist. The cost was $95 + shipping. If anyone is interested, Carver can get them for you. Keller Bros. was the other dealer that had them and I thought they said they had 1.5" spacers. I will post some comparison pictures when I get them in later this week.
Just curious: Can any of you 2800/3400 owners with R4s slide your hand between the fender and sidewall?