They are great snakes and they just get better as they grow!
As for your setup, - I prefer aspen shavings over bark as it is easier to burrow in and these snakes like to tunnel around their cages.
- You shouldn't get a 4 foot cage yet. I would get a cage that is about 2 feet long (a 20 gal long works fine) for the first few years, until the snake reaches 3-4 feet. Milk snakes are escape artists and a big 4 foot cage can often be harder to secure for little snakes like a baby milk. Also, the snake may have a harder time finding its food in a large a cage and you will certainly have trouble finding a baby black milk in 8 square feet of aspen shavings!