Moving from Drupal 8 Beta 3 to Beta 4 turned out to be a significant chunk of work.
Much of that was due to functionality that was previously deprecated but still available that has been removed altogether. We continue to be nudged into doing things the D8 way.
Here are a couple of more changes I made for this release. This wraps up my findings for Beta 4.
Fatal error: Call to undefined function
Function drupal_match_path() has been removed and replaced. For example, instead of the call
In javscript code, drupalSettings.path.basePath has been replaced with drupalSettings.path.baseUrl .
New PathMatcher service
current_path() is replaced by the <current> route