Hiya, I had the same experience with my DD.
We started to potty train her at 3.5 years as I felt that's when she was ready. So glad we waited as she picked up weeing in the potty in 2 days.
Poos were a challenge. She would only poo in a pull up nappy, and I didn't want her to get any more constiptated than she already is, so I just put a nappy on her and told her to stand in her usual corner and do it. I made it clear that nappies were only for poos. She is old enough to understand.
She wouldn't tell me when she wanted to poo as she was scared, so I just used to put the nappy on when I thought she needed a poo (especially if she hadn't done one for a few days), and she usually did it. I think it's better to take it slow, and not make it a big deal and instil more fear in them.
Anyway, eventually (a few weeks later), I started to give her laxatives that the doctor prescribed to make going easier and the stools easier to pass (cannot remember the name, but is a childrens verson of Movicol) - it's basically a gentle stool softener and doesn't make the bowels lazy. This meant that when she weed in the toilet, the poo would often come out as she had the urge to go. She then got used to the sensation and no longer feared it! She did this a few more times, and then had a couple of accidents in her knickers (I no longer put nappies on her), but then she started to tell me herself that she needed a poo, and I sat her on the toilet and she did it!
I made sure I gave her treats when she did a poo in the toilet as an incentive, and got her those fun kandoo wipes. Make it fun for her and don't tell her off for accidents. Just explain that she needs to poo in the toilet, and she's a big girl. She will master it in no time!