Solr4J is a work-in-progress project that aims to embed a full Solr server inside a java application. This is not a Solr API or management tool, only management of the Solr server installation and support for starting and stopping the server, as well as defining core templates using config sets.
For a Java API to use the Solr REST API, use the SolrJ API.
This project is heavily inspired of the MariaDB4J project, which does a similar thing as this project, but to launch a MariaDB database from Java.