I should mention the tire wear is on the fronts, the rear tires have been going strong for 33,000 kms and counting (Bridgestone Playz)..most likely closer to 40,000 as they came fitted out of Japan.
That may be your problem - having different tyres front and back. Even though labelled the same size, it's likely different models will be slighly different sizes, so with the car at a constant speed, they want to rotate at different rate. No problem on a 2 wheel drive, but on 4WD, something has to give somewhere, and that may be 2 tyres slipping ever so slightly all the time, leading to increased wear.
On a 4WD vehicle, it's preferable to replace all 4 at the same time, so all the same size. Then rotate regularly so the wear is the same.