I have a 4010 and a 5' b.h., and would not advise another 4010 owner to use a 5'. It works OK, until you are going up a hill in 3' high weeds, then the tractor is REALLY working hard. I have to take 1/2 a pass under those conditions.
I don't think you will have any problem with a 4' on 3' high grass or 1" saplings. Just remember to BE CAREFUL at all times, but especially on hills.
Also remember that the brush hog can flip up if it catches on something...to the point it can hit you in the back of the head if it is not restrained.
Oh...and get a couple extra hats, becasue as you mow past a tree branch that knocks your hat off, you will have nothing but mouse nest material when your B.H. runs over it!
And your favorite beverage container/cooler/cup...not much left of that either.
Knowledge is something you get right after you needed it... /forums/images/graemlins/blush.gif