Best practices for shared methods in AR models

1.) Create a new class that these 4 AR models would inherit from.

This won't actually work. Rails would try to use the same table to
store all of the models if you inherit from one. See the "Single
Table Inheritance" section from
http://api.rubyonrails.org/classes/ActiveRecord/Base.html

2.) Add a module to lib and include that in these 4 models.

If you don't go the plugin route (which I would probably recommend),
this would be the way to go.