Thwe fourth beam is supposed to be reflected in the water drain which you haven't filled with water yet. For this you need the bucket from inside the shed. Shoot the explosives through the shed's window and get everything from inside.
The bucket must be placed somewhere inside the first pyramid. This will help you raise the 4th statue.
But if the boy don't come out there are probably some hyenas left. Kill them all until you hear the Indy theme melody.
Furthermore, you need something from the digsite, too. Just come back here if you need more hints.