Day 3 or 4 is a common time to lose chicks who have any undetected internal issues because now their absorbed yolk has run out and if they are not absorbing nutrition from their food they can't make it any longer.
Make sure that she's getting properly hydrated. Go ahead and offer electrolytes, but in a separate waterer so that the chicks have access to clean, fresh, PLAIN water as well. Imagine if you had nothing to drink but a Gatorade flavor you didn't like. *yucky face*
You can also make some of their chick starter into wet mash to help them stay hydrated. Chicks go crazy for wet feed!
Pasty butt is often associated with shipping stress and/or with an overly warm brooder.
Here's my experience with a case of persistent pasty butt last spring:
https://www.backyardchickens.com/posts/24506396