Tapbots has been updating its alpha of Tweetbot for Mac since pushing it live a few weeks ago, and today's version 4 is a pretty big one. The most notable change is the ability to view multiple accounts in columns à la TweetDeck: click the gear icon at the bottom right of a feed, select "Open in New Window," and you'll have a separate column that'll automatically snap to the original feed when dragged to its right side. This can also be used to keep an eye on mentions, search and so on without taking your eye off your main tweet feed.
The next most visible feature is a menu bar icon from which you can create a new tweet or go straight to your mentions, direct messages, or feed. Other highlights include a streamlined UI which takes up less vertical space, support for custom URL shortening and media upload services, and improved Mountain Lion notifications that work with multiple accounts. As before, Tweetbot for Mac is available for free while in its alpha stage, but will become a paid app once the developers reach a stable version 1.0. Let's just hope that Twitter supports third-party clients long enough for that to happen.