I really think whether they will feel left out depends more on their personalities. For some children it would not take seeing say over half of the girls dressed up, which is what we saw on our trip. Some children might feel bad the first girl they see dressed up; others may not be bothered at all no matter what.
We did two princess meals on our last trip because we had picked every restaurant we wanted to try and had 2 credits left. For CRT, we had a dinner ADR. My daughter had gone to BBB two hours prior, so she was dressed up, but wore a regular formal gown, not a particular princess costume. It was ankle length. Her appointment wasnt until 4pm though, dinner at 6:40. We redid our favorite attractions in between her appointment and dinner and after dinner we left the park after a stop at Town Square Theater, so she wasn't in it all day. Plus it was raining and not as hot as it would have been if TS Debby wasn't there.
Next morning we did Akershus for breakfast and for that we went with a Disney Princess pillowcase dress I got on etsy because I figured shed be spending the day in it. I had a backpack though so changing was always an option.
I really adore the pillowcase dress. My daughter actually has two, the one she wore to Akershus and a Minnie Mouse one she wore to Chef Mickeys. And pillowcase dresses are really versatile. You can put a turtle neck or other long-sleeved shirt underneath in cooler weather, wear it as a tunic as the child grows. At age 4, your girls could get 3 years of use out of one of these dresses. Here is my dd's dress. And you can't tell from the picture, but each princess' dress has little sparkles on it. She wore it with socks and comfortable tennis shoes.