We are taking our kids on their first trip to WDW at ages 7 and 9, but we have been to
Disneyland in California MANY times, and each age has its own merits. If I had to choose between ages 3 and 4, I would probably go with age 4 for my kids, as toilet training, temper tantrums, and naps were still issues at age 3. Age 4 was just easier overall with them - especially with regard to traveling (we live in Western NY and fly to California to visit family). Flying with a 2/3 year old was challenging, but flying with a 4 year old was much easier.
That being said, we took my oldest to Disneyland at the age of 15 months. We all had a good time. We rode Peter Pan, the carousel, the monorail, did the Tiki Room, etc. - of course he doesn't remember it, but we do. It was delightful watching him dance when a band was playing, and he LOVED Toon Town.
We took him again at age 4 (his brother was 2, almost 3) and it was wonderful. They both enjoyed MANY rides at that age. My older one could ride anything, and Nolan couldn't ride the thrill rides yet, but they both loved everything they could ride. At the Magic Kingdom/Disneyland, there are a LOT of rides without height requirements. They loved Pirates, the Jungle Cruise, Small World, Peter Pan, the teacups... honestly, we had a very full day and they loved all of it.
We went to Disneyland again when my older boy was 6 and Nolan (little one) was 4, almost 5. Both could ride ALL of the rides at that age. Our best trip yet. Some of the "magic" was gone for my older boy (he realizes the characters are people in costumes, etc.) but it was fun to jump on the Matterhorn, Space Mountain, and still enjoy the characters.
We're doing WDW with them for the first time in February.
I can honestly say that each age has its own merits and is fun in its own way. As she gets closer to age 3, you could really start to take a look at what you think would fit her personality more. If she's a late potty-trainer, maybe wait until 4. Or if she's really into the princesses and is past the toddlerish needs (diapers), take her at 3. Every kid is so different at these ages.. my boys were late on the maturity side, so I would have picked 4 over 3.