PREFIX : <http://example/> 

SELECT * 
FROM <data-g1.ttl>
FROM NAMED <data-g1.ttl>
FROM NAMED <data-g2.ttl>
FROM NAMED <data-g3.ttl>
FROM NAMED <data-g4.ttl>
{ 
   { ?s ?p ?o }
  UNION
   { GRAPH ?g { ?s ?p ?o } }
}
