The ability of your CUT to pull a box blade is going to be dependent on what you are blading, and the slope involved. If you are moving light dirt on a level surface, I would think you could pull a 5 foot blade easily. If you are blading a gravel drive up a slope, the 4 foot would be better. I have a JD 3038e which is a little stronger and heavier than you tractor, and I have a 5 foot BB. I actually downsized from a 6 foot blade I had for a previous and larger tractor. But trying to pull a full box of gravel up a slope can almost bog me down. However, it works like a champ on more level ground. I can pull the blade in dry soil pretty well, but putting the rippers down will just about stop me.
The BB is a really useful tool, but I'd go with the smaller blade for most things. It's easier on the tractor and its tires.