Most likely the biological father has to petition.
What a tangled web we weave when first we practice to deceive.
YOur ex husband is the legal father. After four years as the legal father he has earned the right to be the biological father. Ignorance of the law is not an excuse and you have no right to force him out of your (plural) child's life because he is an inconvenience.
Parents should remember 3 things: Love your kids more than you hate your ex; when you have children the relationship with the other parent is until death; your children determine what type of nursing home you end up in.
Nothing stated by me should be taken as giving you legal advice or forming an attorney/client relationship.
Attorney-GAL in Ohio.
I've removed the knife from my back, polished it, and will one day return it -- long after you think I have forgotten.