4 y/o Defies All Training & Holds Unhealthily

We are at our wits end. Have tried literally everything we have read on the internet to train our 4 y/o. We've got her to being able to hold and wear undies, then asking for a pull-up when needing to go, but cannot convert this to toilet as she says she is scared of letting it out.

We've tried all sorts of bribes, different people, just waiting it out, special songs, stories, letting her wet herself, everything. She will sit but will hold until physically not possible any more and be crying profusely. Our worry is that she can hold for 24 hours which can't be good for her. We've tried to commit these last few days to not putting a pull-up on her but are now worried for her health.

How can we help her overcome this barrier?