Ah Gatwick... not my favourite of the London airports (although that is most likely because of the distance from London, the city of my home). Actually, in reality it is quite nice (the north terminal is much more civilized than the south terminal - mainly because the south terminal deals with all the charter and low-cost flights). In terms of good spotting ops, it might be tricky if you don't leave the airport.
I haven't been back since they built the new pier with the high bridge, but I'd imagine the views - if you can get up there - are stunning.
The new airbridge in the North Terminal (I believe that is where continental flies from) is good to look from as it places you literally over the taxiways so you can see most of the airport from there. I had the luck of flying from one of the, more remote, gates at the end of this and the views are spectacular (but afraid I can't remember if it is travelator or not). Also Jsnww81, you are right, the North terminal does have better views than the south terminal thanks to the windows.
If you can see them, Gatwick does have a wide variety of airlines and aircraft (however, the more interesting one's tend to fly from the south terminal - you can get there using the free shuttle train from the north terminal). Oh, one more idea which I have never done but I don't see why you couldn't do it, is the remote gates in the south terminal are accessed by another shuttle train and that area might have good views as well - but not sure if security would let you through.
Hope some of this has been useful - there is a lot to do at Gatwick so the 4 hours probably will go very quickly. If you have any Q's pls do contact me and i'll do my best to help out.
Enjoy England (for the limited time you are here!)