Never done it but it could be done. You would have to unwrap everything and resolder the connections on the most likley aluminum tabs. Should only have to desolder the 2 you are removing as they are wired in series for the most part. You would also have to replace the 6s balancing connector with a 4s. The mAh rating is the same on each cell so you would end up with a 4s 3300 mAh pack.

Personally I wouldnt attempt it as its not worth the risk to me. One goof like slipping with the soldering iron and piercing one of the cells and it could blow up in your face. Even the soldering technique has to be spot on as if you get the connector too hot it could cause the same result as above....POOF!