How to enable inference when querying the triple store?

[Max]

Please consider the following data I inserted into the triplestore:

 

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . 
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . 
@prefix mo: <http://example.com/my-ontology#> . 

mo:Author a rdfs:Class . 
mo:Book a rdfs:Class . 

mo:isAuthorOf a rdf:Property . 
mo:isAuthorOf rdfs:domain mo:Author . 
mo:isAuthorOf rdfs:range mo:Book . 

mo:author1 a mo:Author . 
mo:author1 mo:isAuthorOf mo:macbeth .

If I execute a simple query like

SELECT ?s ?p ?o 
WHERE { 
  { ?s ?p ?o } 
}

it works: all triples are returned.

However, when executing the following query 

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> 
PREFIX mo: <http://example.com/my-ontology#> 

SELECT ?o 
WHERE { 
  { mo:macbeth a ?o } 
}

I don't get any result. I expected 

Book

for the triple 

mo:macbeth a mo:Book

to be returned. 

Why is this not inferred? Do I have to enable this somewhere in the 4Store?

Best regards
Max