Your ratio is of as all that counts is what is below the water line. The end of the pipe must be deep enough not to have a vortex. The only thing I can think of to stop that is an elbow pointed down.
Let me explain my thought. IF you can get the 4" donkey dong to glue to the 3" fitting (it will go over the outside of the bell) and get it to seal. What I would do is glue it deep so the end of the pipe comes past the bell section, then I would pour glue in the grove and let it dry. Put a T in the middle at your high point wit a 4 foot stub pointed up. Glue a coupling on both ends and put on threaded caps.
From the top of the T have a pipe sticking straight up about 4 feet. Fill the system from the vertical pipe. when the water is near the top of that glue on a cap. Glue the pipe and the cap, you should see a bead form as you slip it on. That pipe is your air trap now.
Go to the pond end and keeping the pipe under water, remove the threaded cap. Go to the bottom and pull you P trap in, then remove the cap. You may have to tie a concrete block to it to maintain the P trap.
As far as where the pond end is, put it in deep. Take a yard stick and walk out and drive it into the mud. Count how much time it takes to drop an inch, then you can about figure when you need to check it again.
Break the siphon, just cut the cap off.
