[Harloo]

Knowing this thread is a little old in terms of selling your boat, I still wanted to chime I with my experience for future readers of this thread.
My 97 4000 Express had core rot in the area between the aft end of the bow pulpit and the forward edge of the raised cabin top where it slopes upward from the deck onto the cabin top.
Many 4000 Express units were shipped without a windlass. Windlasses were often installed in the field after the boat left the factory.
Some of these field installations had bedding and sealants that failed due to application errors.
Mine was one.
However, the outer deck fiberglass is thick enough (about 3/8" if I remember correctly) that the rot is not noticeable on such a limited area.
I discovered mine while replacing the windlass in 2017.
Had to remove a trapezoid shape piece of deck from the outside, same area as described above.
Was able to salvage the outer piece and relay it onto the new repair panel, thus saving and reusing some of the nonskid area. 
This resulted in some seams of smooth area that look different from factory, but saved some dough.
No wood in the new core. Hard as rock but I still used a backing plate when installing the new windlass.
The rot extended aft down the side deck a little ways past the cut out section. We dug out a few inches of wood and backfilled that area.
Even with the money saving short cuts it was not inexpensive.
I recommend every owner remove the windlass footswitches and check for rot and address it sooner than later.
While you're at it, remove and re-bed/reseal the screws that mount the drink holders to the cabin top next to the sun pad area.
Had a leak there too!
Good cruising and good wrenching!