In the wild I think brook trout would live well past 4 years. Especially in small mountain streams where their growth rates are quite slow and fish max out at a foot in size.. However, in aquaculture pond like such as yours where they are fed lots of food and have fast growth rates, I could see their life span being 4 or 5 years... I honestly don't know for sure though. I think if you kept a few fish to 4 years, you'd be OK, but past that you really run the risk of mortalities.
That's what I'm afraid of.
