We recently purchased a quad bunk TT and like the layout. It came without any ladders to get up and down from the upper bunks. None are built in either. I know the upper bunk has a 250 lb weight limi...
I made one out of red oak 1x stock I had. Notched the stringers for flat rungs, glue and screwed it, and made hooks to engage the top of the center drawer/cabinet unit so it can be easily removed. In the center, both top bunks can be accessed from the ladder. It works very well.