/update/json was removed from the example configuration in 4.0 because /update now handles content based on content-type internally. It may not be spelled out as clearly as it should be, but here's the CHANGES entry for it:
* SOLR-2857: Support XML,CSV,JSON, and javabin in a single RequestHandler and
choose the correct ContentStreamLoader based on Content-Type header. This
also deprecates the existing [Xml,JSON,CSV,Binary,Xslt]UpdateRequestHandler.