I'm on about the 5th beer aging in these barrels, trust me, it can get over oaked very quickly. The best way to go about it is taste. But 4 weeks will be pushing it. Personally, I find most commercial breweries over age their beers in barrels as well. I pull mind a lot sooner than others. Sometimes, even in a 60 gallon bbl you can pull it after 4-6 weeks.