There are 4 screws: 2 you can see and 2 that are under the pickup. If you attempt to lower the pickup by tightening only the upper visible screws, you will break the housing and possibly render the pickup inoperable.
So, first lower the screws under the pickup to the desired height, then tighten the upper screws to hold the pickup in place. You'll need a nice skinny phillips head screwdriver to get down to the lower screws.
