Figure your trailer at 900 lbs, add in about 500 for gas an gear and you have a down the road weight of close to 4K lbs.
I think you are OK with the Explorer. Is it the V8? That makes a difference.
Once again, you can pull almost anything with almost anything if you want to.
I have a friend who just traded his Explorer at the beginnning of the summer. He was flat towing a Jeep CJ. Total weight was less than your boat. Felt as if the CJ was pushign the rear end of the Explorer (note that when flat towing, the towed vehicle has no brakes, makes a huge difference.) He opted for 1/2 ton Silverado and was incredibly happy.
No matter what you tow with, make sure it is set up to tow. It should have an auxillary transmission cooler at a minimum. Also look for heavier fuses for flasher relays and I also recomend more aggresive brake pads and a power steering cooler.