How did you do level 4?
You have to move the bucket along to the blue door. The tricky part is opening the door then clicking the red handle quickly. The bucket of water lands on the guys head.
I half to be honest and say that my son got the bucket on the blokes head for me as my reflexes weren't fast enough.
After getting the bucket on the blokes head click on the horn and the bloke on the chair will shoot him.
After that it's easy.