This issue with tanks being too big for a bp has to do more with young/hatchling snakes. If your BP is 4.5 feet it must be an adult. Having a tank the size you want shouldn't be an issue, it's that it is just a waste of space really. BP's are not very active and spend the majority of their time in their hides. That is part of the reason why they don't need such a big tank. Having such a huge space will mostly tank up a lot of room, and not be used. The size you have it in right now is fine and you could easily fix it up if you want it to be visually more appealing. If you really want to go with that much unneeded floor space, at least cut your height down. Once BP are adult they basically stick to the ground, so a lot of height is very unnecesary.