I use (part-time, comercially) a 55 HP 4wd tractor with a FEL and a Farmi 351 winch with 165 feet of cable.
I have no problems at all hauling 6 tree length logs with 10-12 inch buts, through 12 inches of snow.
That is, of course, on level ground with a fairly straight skid road and all trees being softwood.
Hardwoods are going to weigh alot more but a 40 HP will handle any "average" sized hardwood but you may have to cut the really big ones shorter.
Yes, a skidder is more suitable but for part-time use or even full-time thinning work a tractor will do as long as the trails are kept level, fairly brush free and stumps level with the ground.
I used to run a Massey (around 60 HP) 4wd for a friend of mine with an Igland winch on it.
I hauled steady 8 hours a day while another guy cut all day.
We worked full time for a few months with it until we were
done and it was very productive.
The FEL is a must, it can carry all your gear and plow logging roads etc... but it's greatest benefit is all that extra weight it puts on the front which makes such a big difference.
A 40 HP 4wd in an average sized stand of wood with good skid trails will suit you fine.
Good luck and play safe.