That ledge under the window carries much of the weight
of the bunk and anyone on it, so i would want it to be strong. I believe on my Scamp
it is epoxied to the outer shell, encased in fiberglass, and covered in rat fur. I am going by my memory of what others have posted- I haven't taken mine apart.
In your case, I would be inclined to make several custom-fitted, wedge-shaped brackets (sort of like buttresses) under the ledge to add strength.
Four hinges seem like overkill, since the ledge, not the hinges, carries the weight
This thread has some details and close-up photos of the Scamp
bunk set-up (see post #4). I think a Boler
is pretty similar. It might be of some help.