I redid this whole room for less than $600! Make sure you take your time with cuts and fitting. I did this with my father in law (my daughter even got to help out!), and it's less than perfect due to some rushing! More details on my blog.
I also couldn't find any 4x4's. Instead of doubling up on 2x4's, I just used one. That allowed me to change the tops of the head/foot boards to only 48" instead of 50 1/2. You can get enough for a head and foot board out of one board this way.
After looking at a TON of bunk bed ladders online, I designed my own. Just some leftover 2x4's. The sides are 47 1/2 inches long, and the rungs are 13 1/2 inches long. It's screwed to the bunk bed from the back so you don't see the screws.