First of all, thanks for posting with photos and a clear sketch!
That is interesting. I have noticed that the (some?) 4500s have that special receptacle molding, but have never seen what's behind it. What I can't tell for sure is whether the "extra wide" seam gap/wood/etc. were put in by a PO (maybe needing to do some wiring?) or by Trillium
at the factory (or just a sloppy fit maybe). Huh.
Clearly you do need a fix though. And I'm guessing you do not have good access to the inside - the large receptacle molding is probably bonded in place, right?
I think I would want to just fiberglass that gap closed. Now, you could try to do something fancier that is removable for servicing the wiring, but if your wiring is fresh and proper (and/or you make it so - I would), then personally, I would rather just glass it closed, have it be watertight, and then IF ever necessary, just cut the glass where you need to get in and re-glass. It's also possible to install a deck plate (removable round plastic cover) on the inside of that molding if you ever need to.
You will need to be tidy with your glassing, but with some care and liberal use of tape (masking type), you should be okay.
Knowing how the belly band is attached, I would be tempted to do away with it and use something else, but yours may be fine, and I know some people like the look (you might want to check for suspicious bulges on the outside which, if there, would indicate rusting rivet-backers.
The nice thing about fiberglass is that you can easily customize the fit/shape, and it will be solid and watertight. One important point is to remember to solvent wash the gelcoat before doing any sanding, to remove mold-release wax (Interlux 202 or etc.)
That really is a great sketch. How does one make them?