Nothing. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
It stays closed.
When you open all 4 tombs, click on any of 2 middle tombs to get entry to lower level.
Both middle tombs lead to lower level, it doesn't matter which one you will open (you may open only one), the other one will stay closed, forever. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />