I think I can answer your questions!!
1) Are there plans for a 5th major theme park?
-NO!! Like someone mentioned earlier, there was going to be a ‘thrill ride’ park featuring the villans but due to budgetary reasons (Disney is famous for that, especially with Eisner in the helm), they are focusing on just rides. New rides include ‘Sorin’ over’ in Epcot, Mickey’s Philamagic over at Magic Kingdom, and ‘Expedition Everest’ (I think) over at the Animal Kingdom, that being THE biggest project. They was talk of adding another country over at Epcot but as usual, cheap Disney=no green light for the project.
2) Are “cast members” cross trained to do other tasks? (ie. if the Jungle cruise is slow that day do they work the Tiki Room).
-They sure are!! Some of them at least. They’re called Rescue Rangers in Epcot, and something else on the other parks and they get trained to do merchandising, quick-food service, and all the other goodies to fill in for people who called in sick. They can work ALL over the park multi-tasking and such. Also, if one chooses, they can get training for other tasks for I was a merchandise host but I got my food handling certificate and thus was able to work quickservice jobs (drink and turkey leg stands and stuff…not that I did it but yea)
3) Do cast members work in more than one park?
-You can but only for overtime. See, once you’re trained at a specific task (like merchandising), it’s pretty much the same all around. I once worked at MGM-Disney at Main street for overtime and that was a great experience. Overtime opportunities are printing out everyday but specific skills are needed in order to pick up that shift.
4) I was told that Disney will close parks due to capacity. Specifically I was told that last July 4th they closed the Magic Kingdom at 10am.
-Magic Kingdom is FAMOUS for doing that. I remember 4th of July, most of December (Chirstmas and New Years Eve) was full by mid morning. New Years Eve is the worst thing ever since even Epcot was full capacity by the time it reached mid day…close to 50000 to 60000 if I remember correctly.
5) What do you estimate it costs to build a major attraction?
-A lot of money. Too lazy to list but just go to http://www.wdwmagic.com/ and you can find out 
6) I noticed that, compared to other theme parks, Disney’s maintenance seems top notch. You never see a threadbare costume or grease on a ride. Does Disney spend more on maintenance than other theme parks?
-They used to, but now with the 3rd accident on the Big Thunder mtn. Railroad over at Disneyland, it’s really obvious that they’ve been cutting back in it. Same with WDW where I saw numerous things (backstage of course) that made me think about the safety of guests.
7) Do the cast members in the shows get paid comparable to what an off-broadway type actor would earn? What about the musicians?
-They do get paid more than regular cast members that’s for sure. Even characters get paid more, especially face characters (cinderella, mulan). Some are recruited from the WDW College program and make about 50cents more than regular interns.
If you want to know more about WDW, just ask and you shall recieve!! I have sooooo much stuff to share if you have any other questions!!
Reoch