Having 4" pipe in 2 of my wetlands, I will never use less than 6" pipe again. I have 3 pig ponds and 5 wetlands with overflow pipes. The 4" pipes are just too prone to clogging. A number of folks here advocate for siphon spillway pipes which may allow you to place the pipe over your existing dam and then cover it with the material removed while cleaning out your pond. Of course, that stuff will be pretty mucky until it dries out. Good luck!