I've had similar low-clearance problems (front and back) with R4's on my DK35SE. My clearance problem is with the ROPS/fender support rather than the fender so the odd contact with it does not cause damage. As already noted in this thread poor clearance to the fender is more serious as contact is likely to cause damage.
The only solution for my fronts was wheel spacers which I added a couple of years ago as described in this thread:
http://www.tractorbynet.com/forums/...hains-dk35se-cab-post4060838.html#post4060838
http://www.tractorbynet.com/forums/...hains-dk35se-cab-post4071972.html#post4071972
I had a lot of problems with the DUO's on the rear. They were rough and difficult to fit. I got them to work by converting them to H-pattern and by adding an extra cross chain pattern across the end gap as shown in this thread:
http://www.tractorbynet.com/forums/attachments/268885-duo-chains-planned-modifications-3.html
My front chains work great with the spacers. However, I still have intermittent trouble with the rear chains and need to adjust them a couple of times through the winter. Due to tight clearance I have to run the rear chains tight and I think that causes the chains to eventually shift while travelling to a neighbour's in high range. The relatively high speed causes the cross links to move out from the tire and the tensioners eventually shift the chain slightly.
The front chains are loose compared to the rears. When roading the tractor I can see about a 1" gap between the tire and the cross chain as it rotates from the ground contact. I believe the same thing happens with the rears but the tight tensioners eventually cause a small shift in the cross chains. I can tell when it happens by the noise of loose cross chain links and, if the shift is at the end gap, the inside closing link will start to contact the ROPS/fender support.
I have finally decided to fix the issue and will add rear spacers, which I should have done long ago. Also, much of my work is on hills so they will have an added advantage of improving stability.
If it does not cause other problems I recommend going with rear spacers. I do not recommend DUO pattern chains for the relatively small tires on our tractors. H-patterns work well but are more difficult to fit. 4-link ladder chains are the cheapest option and simpler to fit. 2-link ladders will give a better ride.
If starting from scratch I would likely go with a low-clearance chain such as Aquiline or Trygg on the rears. They are expensive but smooth and effective.
Here is a link to a Trygg catalog:
http://www.whitemountainchain.com/2016 TRYGG CATALOG.pdf
Page 8 lists Safety Grip 7mm for 12.4-16 tires.
Aquiline MPC 5.5mm chains are advertised for compact tractors with limited clearance:
Aquiline Tire Chains
Tellesfsdal also have chains for small tractors:
http://www.tellefsdalkjetting.no/in_english/agriculture_chains
One place to look for spacers is Bro-tek. He specializes in Kubota spacers and there is good information here:
http://www.bro-tek.com/wheel_spacers/
It looks like 2" rear spacers for your tractor are $280 CDN. I got my front spacers from him and the service was great. I will buy my rear spacers from him soon.