Lynn, Welcome aboard. I also think your 4 inch will be undersized for major rain/ melt events. I have a .4 acre pond that a decade ago I had deepened. It had a 12 inch galvinized pipe that I thought must be sufficient as the grassed emergency overflow showed little use. I had only owned this place for a year and was not familiar with it.The pipe was fairly rotted so I replaced it with a 12 inch smooth bore poly culvert tile. Upon a approx 3 inch duck drowner in 60 minutes this tile was under water and the emergency flow was running overflow also.The old tile was apparently so rotted on the bottom that water was flowing out below it also. I had my local Soil and Water visit and we calculated that I had 38 acres of hilly watershed feeding this small pond.It was advised to add an 18 inch to it if I did not want the pond to over flow even the grass spillway.The main problem I had was that the water would circulate clockwise in the pond eroding the banks while waiting to enter the pipe and the spillway could not prevent this either.I then added a "pair" of 18inch tile spaced on both sides of the 12 inch approx 30 ft and all 3 tiles dump into a common point into a creek.I set the 2 18 inch tiles 3 inches above the 12 inch so that the 12 inch does most of the pulling and also put a 45 degree on it and submerged it 5 ft under the water so the smaller fish are less likely to be pulled out.My 1.2 acre pond that sits some 18 ft higher than the .4 acre pond has no overflow pipe.That pond was designed to have very little watershed and is maintained with a well. When it does get too full I have a 4 inch deep 12 ft wide grass depression that allows the water to flow down a hill into the lower .4 acre pond.Long story "long" is to do your research and decide if your going to design it for that once a year event of just the avearge inflow.That once a year event happened 3 times last year and these outlets have only run 60-70% since installed.