Tapbots has Tweetbot 4 (iTunes Link) for the iPhone, iPod Touch, and iPad in the iTunes App Store for $4.99 (regularly $9.99) for a limited time.

Tweetbot is a wonderful Twitter client that’s powerful and a joy to use. Sync your timeline across your iPhone, iPad, and Mac (Tweetbot for Mac also available in the Mac App Store). Mute tweets by users, hashtags, tweet sources, or keywords (including regex). 3rd Party support for image, video, read later, and timeline sync services. Tweetbot also has all the features you’d expect in a full-featured Twitter client like lists and direct messages, and others you wouldn’t like the statistics view, a night theme for low-light reading, and much more.