I echo the suggestions to look for cotton options in characters she may be excited about. Between Kohls, Walmart.com and etsy I had cotton ones in characters I thought we'd meet and she wore them all day with shorts underneath.

My kid would 100% have been upset at 4yo not to have a princess dress to wear once she saw even one other little girl wearing one. She likes dressing up in process dresses a lot, though, and has several at home. Most are not even Disney princess dresses, she's just really into playing princess.

Princess phase didn't end until 7, and the princess costume (with other clothes to change into just in case) would have been a necessity for that whole era at Disney. At 4 though, it would have potentially caused a meltdown that would ruin the whole day, whereas as she got older she would have been able to handle disappointment better.

So yes, I would pack one for the trip. Just pack one you have or order an inexpensive one online. Frozen is extremely popular for this, though watch out because some of the Elsa costumes are not great for walking around (long sleeves, trains). A cinderella dress is a bit easier because of the short sleeves. Make sure she's not tripping over it.

Ours loved wearing one for some princess dinner we booked. Not for anything else. They are adorable pictures that are well worth it (now that the kid is a teen).

We're going tomorrow and I am not even putting this on the table. Do not want to deal with the whining hah. My kids have some coordinating Mickey/Minnie shirts from Kohls.

My kid spent the entire time wearing a Jasmin costume - the short top and the pants.

My 4yo isn't really into the princess thing. We got her a dress off Pressley Couture (https://presleycouture.com/collections/princess-dresses) that she ended up wearing half a day at Magic Kingdom. I liked it because it wasn't scratchy polyester, and she wore it to school a few times too.

Mine wore Elsa

We dodged the princess thing. My dd was just not into it. She hated anything with ruffles. Loved wearing her older brother’s hand-me-downs. Went to Disney and had a fabulous time.

Tell you MIL that your kid will wear whatever she wants.

My MIL and SIL kept asking if my 4yo wanted to do that bippety boppety boutique thing and I kept having to tell them no. Has to be the most insanely comfortable knit dress to get DD to wear a dress to begin with. And she’s not really into the princesses. They did not believe me because our niece loved it at that age but I illustrated knew DD would not enjoy it. She wore sone fun Disney tshirts and never mentioned princess dresses. We saw some but not a lot.

Google “cotton Disney princess dress.” As a kid with sensory issues I really wish there were as many options then as there are now!!

If I had a little girl, I would be so tempted to dress her up like Princess Anne at Badminton in the 1970s.

Some do, not all of them. You can always buy one there if she begs.

Yes she did. My son wore a costume too!

I went with my niece to Disney when she was 4.5. She was deep into a dress phase from about 2-6 and didn't ever have a Disney dress. She didn't mention it while at Disney World either. She wore jumpers or shortalls (I don't know what they're called). Never said a word about wanting a Disney princess dress. I didn't notice tons of other kids wearing Disney dresses.

My girls did tshirts with their favorite characters the first time. When they saw the princess dresses they were intrigued. For our second trip, i did shorts and t shirts made to look like their favorite princess dresses. For example, one loved Rapunzle so I got purple shorts and t shirt and added pink and white ribbon embellishments in the same
pattern as Rapunzle’s dress. Same thing for Snow White. Look for inspiration on Pinterest if you think she’d like that.