Spatial4j is published to Maven central under these coordinates (to include source, javadoc, and tests):
<dependency>
<groupid>org.locationtech.spatial4j</groupid>
<artifactid>spatial4j</artifactid>
<version>0.8</version>
</dependency>
Note that Spatial4j marks its optional dependencies as optional (e.g. JTS) so that Maven does not transitively include them. Thus you must explicilty add any dependencies for them as they apply to you.