Doc -- As in most questions asked here, it depends on how you use your tractor, what you are doing at any given time, and the conditions under which you are doing it.
With just 45 hours on my tractor, I notice the front tires are showing wear. Reason? I need 4wd moving loads around the property, and do a lot of that moving on my hard gravel over packed crusher run driveway. I transition from that hard surface to soft/semi muddy ground with every load, so leave it in 4wd, but the driveway is wearing my front tires. . I could run across the lawn instead of the driveway, but that tears up the grass in a hurry with Ags mounted on a 4400 lb tractor with 800 or so lb in the bucket.
I don't mow, but if I did, I would try 2wd first since the tires are on top of the ground, not pushing through soft plowed ground. If it was wet enough to need 4wd, I probablyh would stay off the lawn, even with turfs. If I was doing real farm work -- plow, disc, cultivator, etc., I would probably use 4wd for the traction increase and to spread the pulling load over more drivetrain components.
Many tractor 4wd systems are front wheel ASSIST and not true 4wd like in a car or truck. Even there, many different systems are around, but relatively few are intended for full time 4wd operation -- Subaru is one I can think of offhand -- since it simply takes more energy to spin all those gears and shafts when 2wd will get the job done.
My opinion is to use 4wd only when it is needed, as indicated by the rears spinning or the fronts plowing sideways when you would prefer to turn. You'll very quickly learn to recognize the conditions under which 4wd will be of benefit, whether for braking, turning, or pulling. Any other time, use 2wd.
