Irene Ros (@iros), the Director of Data Visualization at Bocoup has written a very helpful presentation and summary of the changes of D3.js version 4.
Some of the changes are include namespacing, canvas rendering, selection, transitions, text formatting, default locale, brushing amongst others.
Ben Foxal has also written a helpful blog post on the changes required for converting visualisations in version 3 to version 4.