I would say stay away from the 4" wheels. So many more option with 6" wheels. Better brakes, too.

If you already have the 8" Douglass ATV wheels setup on your plane stay with that if you like it. I hated mine. The brakes were terrible. I've been much happier since the conversion to Grove wheels and brakes. I can actually stop my plane if I want, now. I couldn't with the original Matco-Douglas setup.