All of our aviary runs are 6 foot high to make it easier to access. You want to consider reseeding, cleaning, tilling, liming, collecting the odd egg that is dropped in there, catching an injured bird, etc. We also get deep snow, so it is important that we can get inside to shovel on occasion. It also provides a nice place for the birds to stretch their wings for smaller, more flighty breeds. Heavy breeds do well in 4 foot too. The higher it is the more fun you can make it for your birds as well. We have wild blueberry bushes in some of our runs, keep them trimmed and let them grow naturally. The birds love to jump and try to pull blueberries off of them. If they can't reach some, we go in and pull the berries and hand feed them. I guess it's just really nice to know they have lots of open space on days they have to be in the run instead of free ranging.
Jody