How were you planning to divide it? Horizontal or vertical? A four foot tall enclosure is taller than it needs to be for a bearded dragon. Floor space is more important than height, though some height is good. Dividing it vertically would be best, as you would have a 8'long by 4' wide and 2' heigh tank which is really spacious for a single dragon and great for several females together.
Another drawback of having it too high is it makes it harder to get the UVB source close enough to the dragons to fully benefit them. the mercury vapour bulbs do up out higher amounts of UVB at greater distances than regular flourscent tubes, but I beleive it still has to be within 24 inches (2 feet). A two foot high cage would allow you to bring basking areas to around 18 inches of the mercury vapour bulb and still give the dragons some height off the ground floor (ie 6 inches). Also, are the bulbs going to be sitting onto of a screen lid or attached underneath the lid? Could be a difference of 2 to 4 inches.
I have a 5 foot long, 2 foot wide and 2 food high cage built for my bearded dragon....unfortunately no pictures of yet, hopefully sometime in early July.