A few tips:
1) If you decide to stay for any of the night time shows (Fantasmic, fireworks etc...) I strongly suggest you stay put when the show is over instead. Even though you will be tempted to just go with the mob if you are exhausted, sitting on a bench for 15 minutes can make a huge difference. It can be hard to maneuver through the crowd with a stroller and even harder to keep track of 4 kids even if they are really good at staying with you. Let the mob go first. You won't get back to your resort much, if any, later.
2) Do your best to prevent meltdowns -
-Find out ahead of time what everyone's top choices are. I always ask my kids for their top 3 must see's/ do's and promise to work those into the schedule. But, they in return promise to trust me that we will see it even if that means passing by a long line and returning later.
-Make sure everyone gets enough sleep.
-Carry some small snacks for when someone needs one. Toddlers in particular seem to be hungry at the most random times and a little snack can avoid a meltdown.
-Make sure to have enough down time - the pool, resort arcade or watching a little TV in the room can refresh everyone.
3) If the older kids will be riding some of the rides without you, make sure they know where/ how to meet you after. Many rides have gift shops which make good meeting spots. My kids have never been questioned when riding without an adult, but some on here have reported that their children have been asked how old they are. Yours are old enough to ride without you, but you may want to let them know that the CM's may ask. Expedition Everest and Rock'n'Roller Coaster only have 2 seats together, but on the other rides with height requirements, they will be able to fit 3 across. Also, use Fast Pass for these times so the time they spend in line, and the time you spend waiting and maybe worrying, will be minimal.
4) Make sure they know what to do if you become separated from one another.
5) Establish a plan for dealing with meal times. Maybe the 12 year old can find a table with the 2 year old while the others help you carry food or something similar.
6) Get most things ready for the park the night before - have the bag packed, have the kids clothes already laid out (some people pack individual outfits in zip lock bags which might help), have breakfast in the room if it is easier than navigating the food court crowds in the am
I think you will be just fine navigating the parks and resorts on your own. The only places I personally would avoid would be the water parks. They are very difficult to navigate if you are the only adult with a range of ages. Your 3 older kids would not find any enjoyment in the kiddie area, but only the 12 year old is old enough to be alone at a water park so you can't really let them go off on their own. And if wouldn't be much fun dragging the 2 year old around to all the water slides. The 2 year old could actually go on most of them, but lugging the double rafts up by yourself is exhausting. And, it would be too hard to keep track of everyone in a lazy river.
Have a great trip!