We've done the Erie canal, GAP & Pine Creek, don't write off the Pine Creek. There is a campground mid point, Pettiecoat I think it is called, it is right on the trail, they have real toilets, activities, etc. They also have glamping tents if you don't want to haul a lot of gear. We've used Pettiecoat for a weekend of out & backs on the Pine Creek trail, it was very nice. I know this doesn't fit the 4 day thing but it could also be a good starter ride before committing to a a longer haul on something like the GAP or Erie.

Also someone suggested DC for a spoke tour, it has a huge selection of interconnected trails in an urban environment, again not what you were asking about, but an experience none the less. We used Crystal City as a base since its an office town, i.e. the place is pretty empty on the weekend and there is a direct connection to the trail network there.

You might want to check out Jim Thorpe PA and the Lehigh & D&L trails, or the D & R canal in NJ, again not exactly something you can go 4 days straight, but you could get 20 mile segments.