What if my opponent ALREADY has a creature with power 4 or greater in play when I play Hidden Predators?
Hidden Predators has what is called a 'state trigger', this means that it looks for a specific thing to be true and then it triggers. So if your opponent already controls a creature with power 4+ when Hidden Predators enters the battlefield it will see that the game state it is looking for is true and trigger. Hidden Predators will then become a creature when the ability resolves.
603.8. Some triggered abilities trigger when a game state (such as a player controlling no permanents of a particular card type) is true, rather than triggering when an event occurs. These abilities trigger as soon as the game state matches the condition. They’ll go onto the stack at the next available opportunity. These are called state triggers. [...] A state-triggered ability doesn’t trigger again until the ability has resolved, has been countered, or has otherwise left the stack. Then, if the object with the ability is still in the same zone and the game state still matches its trigger condition, the ability will trigger again.
Then Hidden Predators will trigger and turn into a creature when the trigger resolves. The card doesn't mention anything about playing a creature, or creatures entering the battlefield, or anything like that. If the opponent controls a creature with power 4 or greater, Hidden Predators will turn into a creature.
Note that this also applies if creatures on the battlefield get pumped up. If my opponent gives his 1/1 creature +3/+3, he now has a 4/4 which triggers Hidden Predators.