I would not reccoment using 4-HI on wet roads unless the roads have lots (as in basically flooded) of standing water on them and travel under 35mph when doing so.
4-HI is for low traction conditons like gravel, dirt, snow, ice, mud etc. Yes wet roads have decreased traction over dry roads but its still enough traction to put un-nessescary additonal wear on your tires. Your front diff is also not lsd so locking them into our driveline (both auto and manual locks) creates additonal wear arrond turns.
from a mechanical standpoint I belive the reason 4wd creates additonal wear is because I think the t-case puts more power to the front wheels so that your vehcle can pull you outta tight spots with the front rather than have the back wheels push you in deeper. low traction surfaces give enough to not put too much wear on anything but on paved roads this would create the effect of your front weels pulling the back wheels faster than they want to spin.