4x4s? 1" thick wood by 8" long cut to the length of the cage works fine. I'd do at least 4 larger shelves (2 that run the length, 2 that run the width), a bunch of smaller ledges and sitting spots, a tube, a hide house, either one that hangs or one that goes in the bottom of the cage. Fleece hammocks. You can make it look awesome with a little TLC!
If you're feeling really adventurous, you can make one "middle level" shelf that spans the entire middle with a jump hole cut out, and that would help sturdy it up as well.