I hope you have a good emergency spillway. A 4 inch pipe is not nearly big enough as a primary spillway, but is big enough to use to control the normal water level. What you need is a hybrid bottom siphon / surface spillway. You will want it to be dry during freezing weather, so a valve at the top, but slightly on the downhill side and one at the downhill end will be necessary to control and start / stop the siphon. You would close the valve at the open, downhill end, open the valve at the top, fill the pipe on the downhill slope, then close the valve at the top and open the one at the downhill end to start the siphon. You would need a wye in the pipe with a valve at the high water mark that would be normally open to keep the water level where you want it. When not in siphon mode, all the valves should be open. In siphon mode, the top valve and the one at the high water level would be closed. The downhill end of the 4" pipe should be considerably longer than the end going into the pond.
If I can figure out how to put a drawing on here, I will. Edit: I took a picture of the drawing with my phone. Don't know how I would post a pdf otherwise.
