The scope is 3-4 inches above the barrel, so it's then perfectly normal for the BBs to hit 3-4 inches lower since they are parallel. Make sure the windage is properly adjusted, but elevation has to be adjusted over a longer distance.
There is a known problem with the CA G36 integrated scope. It will move when you turn the adjustment screws clockwise, but sometimes not come back in place when you unscrew. I think there is a way to "unstuck" the scope using your fingers, but I never had a problem with mine so I don't know.
The hop-up gives a backspin to the BB to make it travel farther. It's not always noticeable indoor. You should leave it turned off (you can turn it on a little if BBs roll out of the barrel). When you go to a game, you can then adjust it to get the best possible range without shooting BBs upward.
My CA G36 hop-up wasn't working when I got it because it lacked the tiny cushion that pushes down on the rubber. Try to look down the barrel with a flashlight and see if there is a difference between hop-up turned off and all the way up. You should see a black bump.
Remember that even if your scope is perfectly zeroed, BBs only have one tenth of a real bullet's velocity. Their path is not always predictable and they are affected by a bunch of elements.
