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.

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.