My 4 year old daughter knows how to pee in the toilet but when its time for her to poop she goes in her diaper. she has another year of preschool and soon its off to Kindergarten. We do ask her if she need to go potty and she does only with the number 1. We only get frustrated when she doesn't tell us that she has to go poop and we find out that she pooped on herself. I know its still a work in progress with the potty training for her. when she does go pee in the toilet she is always saying that she doesn't want to fall in the toilet we do have a toilet set for her to use. What type of advice should we do so that she can go poop in the toilet.
Do you have a toilet seat insert or a toilet seat that attaches? We have tried both but the attached kiddo toilet seat, like this one (they sell them all over the place), is much more secure feeling and not very hard to install on one or two toilets in your home. After a pretty short time using it, my son got comfortable enough to use the full size seat as well.
There is also the possibility that your child's complaint about the seat is more of a distraction and there is some other reason (discomfort, difficulty, boredom, etc) which is very common in kids that age. The most important thing is to keep an eye on them (look for cues) and keep a positive attitude about using the toilet, even though it can be immensely frustrating at times.