Possible changes 

Possibilities for discussion.

1. Quads are not extended Triples (useful if the default graph is a union of named graphs but this isn't always the case) but share a common supertype.
1.  Add a CONTRIBUTING file
1.  3 parts of a Literal
1.  getObject is a Resource, not a Literal
Sesame-4 will depend on Java-8, so I am fine with the dependency here.

Optional should reduce code for the main, as you will always need to test "!= null" anyway in safe code, so the .isPresent before .get should help to formalise the process, based on my limited Java-8 experience. In this case it won't always be relevant, but Optional.orElseGet and orElseThrow are also quite useful with Lambdas.