I would say go for it! I had a great experience bringing my 4 month old to the world last December. We booked last minute, which was great because if I had thought about it, I probably would have thought it would be too difficult. He slept in the stroller when he was tired, he went on any ride he was allowed, and had no fear. He loved the characters, and we had a great time. Whatever you decide, it will be the best for you 
Like a PP said, she just hung out in the front pack the whole time, went on all the rides with me ( sometimes she was sleeping and didn't even know she was on a ride ). I wouldn't hesitate to bring a baby who was 4 mo. They are much easier than toddlers, I'd much rather take an infant anyday.
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. It was very hot for early June and I remember just about every CM we passed sprayed cool water on us with those mister fans an I breast fed so it was really nice having the stations to get out of the heat and into AC every few hours. 
Both of my kids started to get into a regular nap schedule between 4-5 months (I am very protective of their sleep schedule). If yours isn't on a strict schedule and/or can sleep in a stroller I say GO FOR IT!
A 4 month old doesn't count in room capacity, doesn't require admission tickets, You don;t need to buy food (unless you are doing formula). All you need are diapers, wipes, clothes, bottles, stroller and big sunshade. Seriously if you saw the list of things I'm bringing for my almost 3 y.o and 1 y.o, this looks like peanuts! Disney provides cribs or pack & plays, you don't need a car seat if you use ME.
Your older child will have a blast and the your infant will just go with the flow.
It is so much easier now than when they're crawling/walking/talking back
Enjoy your trip!
WOW! I'm impressed that you were feeling good enough to walk around WDW after only 8 days! That's great!
We took DD when she was 6 months ( sitting up but not eating solids yet) She was SO easy. Much easier than her two brothers
I felt the same way *almost* after my 2 yr old was born (homebirth with midwife). I was actually dying to go on all of the rides that I had to walk past during my pregnancy LOL. I was hoping to go into labor at Disney and get picked by the dream team to stay in Cinderella's suite and birth him there LOLOL... but no such luck
haha... After the 4th within 5 years, they just kinda fall out (okay, im only kidding) but he was our unassisted home waterbirth and I was feeling back to normal within hours
Are you on any UC boards? (I haven't UCd but I should have, things would have gone much better. 
)You are my heroine.
OP, I haven't taken a baby that little, but I figure, as long as you're not doing it FOR him, as long as you don't expect him to remember it, and as long as the adults are willing to switch off with him to sit out any bigger rides, I'd go for it.
But I'm also coming from a place of we didn't leave our guy with *anyone* for a very very long time. DS nursed exclusively until he was almost a year, so I was irreplaceable, and we had no one trustworthy for even an hour's worth of watching...DS was 3 months for a train trip to Oregon for DH and my 1st anniversary and we walked all over the city to pick up our wedding album. Traveling with DS is second nature to us, so we wouldn't even think about it twice, when we have another baby, to take DS and a baby to Disney whenever we wanted to.
We took our 7 week old last September. She was the best behaved child in WDW. She went on lots of rides, and gave me no trouble. Now my 5year old.. thats another story all together.
