Looking to use Twitter4J to download the full 280 characters of extended tweets? I can easily be done. You need to have a relatively recent version to enable extended mode, I have tested this with version 4.0.6 and it works well.
Its pretty straightforward and you enabled it at the configuration builder phase.
twitter4j.conf.ConfigurationBuilder()
.setOAuthConsumerKey(""))
.setOAuthConsumerSecret("")
.setOAuthAccessToken("")
.setOAuthAccessTokenSecret("")
.setTweetModeExtended(true)
Thats is, the setTweetModeExtended(true) is all you need.