If you don't control a creature with power 4 or greater immediately after Ornery Dilophosaur attacks, its ability doesn't trigger. If you don't control one as the ability resolves, it has no effect. It doesn't have to be the same creature at both times, however.

If Ornery Dilophosaur's power is raised to 4 or greater, its ability triggers when it attacks.

Ornery Dilophosaur gets just +2/+2, no matter how many creatures you control with power 4 or greater.

Once Ornery Dilophosaur's ability has resolved, it keeps +2/+2 for the rest of the turn even if you no longer control a creature with power 4 or greater.