Brian,
I really think it's all about what you'll be using it for. I have a 48" on my BX25 and plow over some pretty think brush, acres of blackberries, vines, tall grass, thick weeds, small trees (2-8 ft tall) etc.. As I clear the edge of my manicured space, I get into pretty tight places in the trees. I wouldn't want a 5 ft'er in my case. The maneuverability of the 4 ft'er far outweighs the extra width in the field. If you're going to just maintain a field, I'd say a 5 ft'er would be fine.