I didn't visit the parks pregnant, but I did go on a
Disney Cruise in my second trimester (with my third child). I'm one of those people that gets morning sickness, but it's all-day sickness, for the entire 9 months. I'm envious of women with easier pregnancies! If it were me, I would postpone the trip--your DD is young, she'll get over it. You have no way to predict how you'll feel at trip time, and I'd hate to see you go through all that planning and effort and expense, only to wish you'd gone to a beach or something instead. Which might actually be a good idea--go some place relaxing, where you can nap or take it easy as needed.
If you do choose to go, just plan to go at an easy pace. With a 4yo, you probably wouldn't be going rope-drop to fireworks every day, anyway. And one nice thing about WDW is, it's very easy to find a bench and a smoothie place or whatever, relax, watch the sights, and recharge.
FTR, we also went to WDW with a 6mo. It was easier than you might think. We used a baby sling, DH carried him most of the time. We had a stroller as well, but the sling worked best for us--he could watch the world at a higher level, and when he got tired/fussy, we kind of pulled the sling up to give him a little cocoon, blocking out the sights, so he could sleep. We could do most rides with him in the sling (obviously not the thrill rides, but there we used the baby swap).