ipywidgets 4.0 Release Document

In theory, ipywidgets 4.0 is supposed to have the same API as the IPython 3.x widgets. ipywidgets 4.0 was intended to only be the action of splitting the widgets out of IPython into their own repository. However, the import path has changed from

from IPython.html import widgets

to

import ipywidgets

Also, custom kernel authors will need to add a full state push in the comm open message for widgets and a comm_info comm. See here and here