Postulanti Ostinati

Advisors that you haven't controlled continuously since your turn began may be tapped to pay for the second ability of Persistent Petitioners. This includes Persistent Petitioners itself on the turn it comes under your control.

It's not possible to tap one Persistent Petitioners to activate both its first and second ability, or to activate its second ability and that of another Persistent Petitioners. This means that if you tap four Persistent Petitioners, the target player mills the top twelve cards of their library, not the top forty-eight.

The last ability of Persistent Petitioners lets you ignore the "four-of" rule. It doesn't let you ignore format legality. For example, during a Ravnica Remastered Limited event, you can't add Persistent Petitioners from your personal collection, no matter how much they ask.