Can another sibling apply to be a guardian for an elderly parent after a guardianship has been formed with 3 other siblings?

You have to show up for court proceedings. If you did not show up and have since changed your mind, forget it. The judge takes a very dim view of indecision.

If a court has awarded guardianship to someone, I think it would be difficult to have that changed unless you can show the court that the person or persons already appointed are incapable of serving or are not acting in the best interests of the patient.