I do some excavating and small pond building, in your situation I would consult an engineer to run a few numbers and find out what size overflow pipe you need, there are a bunch of variables such as how big an acreage are you collecting in your pond,
A 4" corrugated drain tile is not big enough to drain anything, and is more then likely crushed to half capacity or less at anyplace along its length, just from stepping on it wrong.
