The last ability of Relentless Rats lets you ignore the "four-of" rule. It doesn't let you ignore format legality. For example, during a Bloomburrow Limited event, you can't add Relentless Rats from your personal collection.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to one Relentless Rats may become lethal if another Relentless Rats leaves the battlefield during that turn.