We just returned from a trip with a four month old a couple of weeks ago. She did TERRIFIC! I didn't find the tummy carrier/sling overly helpful. It was too hot to wear it some of the days. She's a little baby for her age, so it wasn't a problem just carrying her in the lines, etc.
A few things that worked great for us:
The baby care centers were terrific. I nurse and bottlefeed, so I used them for nursing her on a rare occasion, but mainly for heating up her bottles. She was quite picky about her bottle temperature and once we found the baby care centers, she ate much better for us. My only complaint about them was that they weren't always located in a great spot. I never did use the one at Studios since it was located at the very front of the park.
Someone on the boards had suggested making up diaper kits each day. We took four of them to the parks each time we left. We simply put a diaper, a disposable changing pad, and about 5 wipes in a ziplock baggie. That way, we could just quick grab baggie and go into the bathroom to change her rather than having to take the entire diaper bag in.
We took naps almost every afternoon on our 9 night trip. This gave me a chance to nurse her mid-day and for all of us to relax. We would then stay up until midnight or 1 a.m. at the parks and everyone would still be in a good mood. I also have a 4 and 7 year old, so they needed the naps as well.
At the counter service restaurants, we were able to bring her stroller into each one while we were eating. So we didn't have to worry about a highchair there. At the table service meals, we would bring her carseat that attaches to her stroller and just set that on a chair. Since we usually came back for a nap, we would only take the carseat for that part of the day. It was rare that we saw a restaurant with the infant seat high chairs, so we were very glad to have our carseat.
Our four month old is a very calm, happy baby. She enjoyed the characters and absolutely loved the rides and shows. In fact, she would cry each time the show or ride stopped! It was quite cute! As soon as a show or ride would start...even if she was really fussy...she would immediately stop fussing! It was great! We had extended family with us, so we would all trade off staying with her while we rode rides that she was unable to do. There were so many that she could do though, so that was fun. We took advantage of the baby swap passes, which were a nice option for us also.
Good luck with your trip. I would not hesitate to take a small baby again. It was a blast and much easier than I had anticipated!