Smaller Couples Trailer with a Slide Out + Queen Bed

The wheelbase is nearly irrelevant. The overhang and suspension type has much more effect on stable towing.

4500 is a bit light for something like you describe. The Cougar has what you want but is much, much heavier.

You'll likely be looking at units without slides, and that's a good thing, but it also means a climb-over bed.