I have an 04 disco with 116K ... I want know some info on towing. I'm going to towing a truck with a curb weight of 3.9k and with the U-Haul Auto Transport Trailer. Its going to be a 3 hour trip and the max speed.. I would be going is like around 55-60 Mph. Wanting to know if this is alrigh to tow and what should I check before I towing.

You're going to want to be sure that the trailer will have brakes on it towing that much weight. If it's from U-Haul it should have surge brakes and that will be good because you probably don't have the trailer wiring plug set up for electric trailer brakes with a brake controller in the cab. Any, the disco won't set any speed records going uphill towing that much weight.

Make sure you don't have any cooling system issues beforehand. If you do, you'll find out 20 minutes into the tow.

You should be fine otherwise.

you will also need a trailering wiring adaptor specifcally for land rover

You are well within the towing specs as laid out in the owners manual. Towing with the Disco....you can tow 4400lbs with a tongue weight if 330lb with storage area loaded (550 lb empty) with a TRAILER WITH BRAKES in high gear at highway speeds. As others have said though, do a pre-tow inspection and make sure your brakes and cooling system is in good shape before you set off. As long as all that criteria is met you'll be perfectly fine.

When I'm pulling my Wakeboard boat, which is pretty heavy, I find that my 4.0 Liter DII shifts into last gear too soon and in turn lugs the engine badly. So I change to sport shift mode so that it winds each gear out further before shifting, and I run the Disco in "3" instead of "D" until 3rd gear gets wound out really good, then shift it to D. Usually only get to that speed on the interstate though. Make sure your running premium fuel as well. You having that extra 0.6 Liters will help you out too. Check you coolant level for sure.

I would tow it in '3' the entire time and forget overdrive. I've always done that on other vehicles when pulling decently heavy loads. Doesn't affect mpg enough to notice and helps keep the trans temps down.

Keep the trans in 3rd gear, set your cruise no faster than 65mph for a experienced trailer tower, no more than 60mph for everyone else.
Other than the lighting adapter your fine.