Yes there is such a thing as too much time... I think a lot of it is really what you plan to get out of it and everyone in your travel group is not going to have the same desire to be there a ton. for us, with mk usually the latest opened park and generally just in the mood for somethign you only find at mk we end up there a lot. Now that doesn't mean we'll spend 4 full days there but generally we'll be there almost daily for at least an hour. I think a lot has to do with why your there. A family isn't going to put up with doing all of mk every day for 4 days, someone is going to be done really fast, someone a little later and generally that last day only the fanatic is actually enjoying going yet again. But there is ways to do MK different. Sorcerors of the magic kingdom for example could be the goal of one of those days. It'll take you a good 6 hours to play and you'll see MK like you haven't in the past. Now the pin trading games are starting to reappear if you have the funds to do them, but they are much shorter.. The pirates stuff is a good part day another day...
yes you can burn out but it really depends on how you do MK on each trip. If your doing the same lets go on all the rides goal 4 days is too much in my opinion. Set your goals more specific and you have a mission or reason to be there and it'll change both how you see the place (smelling the roses and looking up more).