[JRTJH]

The OP's photo (in post #1) is at an "odd angle" so it doesn't depict how much (if any) that the slide fender overlaps the stationary part (if it does overlap) however, judging from the thickness of the slide corner molding, there's nowhere near a 3 or 4 inch gap. Possibly 1" or 1.5" at the maximum. A couple of clearer photos, taken at an angle that doesn't "accentuate the problem" but that give a more "straight on" view of how the trailer fender actually looks would be helpful.

In my view, even from the angle that the photo was taken, there's clearly not a "3-4 inch gap"......

ADDED: As suggested earlier by another member. Find a couple of the same model trailer and look at their fenders. If they're the same, then it's how the factory produces them. If they are significantly different, then make a decision on which is correct and which is not correctly installed.