In 7.x-4.x twitter.lib.php has been removed from the module. Rather than simply installing the module, users must now additionally install the Libraries module and obtain the twitter.lib.php which is hosted offsite on Github.
I think this complicates the install process, when the twitter.lib.php could easily be dual-licensed MIT/GPL and restored to the module codebase. Doing so would simplify the installation, reduce points of failure, and reduce the amount of support traffic we see in the issue queue.
I don't understand the reasons for removing twitter.lib.php from the module, so I'm submitting a patch for its restoration, and hoping this leads to some discussion.