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.
I concur with Keith on this. I currently have a Woodinville 8 gallon barrel on it's second run with a Wee Heavy. The first run was an Imperial Porter for four weeks. I was worried I would get too much barrel character. While the wood and bourbon are assertive, I'm really happy with the results.
The Wee Heavy I'm going to leave for a minimum of three months to get a better idea of volume losses and character from extended aging on a second run barrel.