Minimum viable population is less of a rule and more of a guideline. Specifically speaking, it's usually defined as something like "the minimum population to have a 95% chance of survival for 100 years, in a population that isn't being specifically managed by people."

95% is a pretty high percentage, so there's a lot of "room" below it. Maybe 4 moose only have a (to pull a number out of the air) 25% chance of survival....but that's common enough that it isn't surprising to see it happened.

And furthermore, populations can have big advantages that mean their actual odds of survival are higher than the given percentage. For example, moose introduced onto an island with no competitors and no predators can expect to have a greater fraction of their offspring survive. And that means their population will more rapidly grow out of the danger zone.