For AirPods (including AirPods 4), Lost Mode in Find My doesn’t “lock” the case the way Activation Lock works on an iPhone or iPad. Instead, it’s more of a "tracker and notification system" for whoever finds it. When you put your case into Lost Mode, you can add a custom message and phone number or email so that if someone else with an iPhone brings it near their device, they’ll see your contact info pop up. It also lets you track its last known location and get notified if it’s found or comes back online.
As for compatibility — unfortunately, the AirPods case itself isn’t permanently tied to your Apple Account. That means someone else could pair your lost case with their own AirPods if they reset it, and Find My won’t stop them.