4store looks to be a good solution however the documentation is pretty sparse at this time and when I last looked at it there was no ability to delete an individual triple from the graph.
I would also take a look at BigData
Here is a quote from their main page summarizing their offering.
Bigdata(R) is an open-source scale-out storage and computing fabric supporting optional transactions, very high concurrency, and very high aggregate IO rates. Bigdata was designed from the ground up as a distributed database architecture optimized for very high aggregate IO rates running over clusters of 100s to 1000s of machines, but can also run in a single-server mode. Bigdata offers a distributed file system, similar to the Google File System but also useful for workflow queues, a data extensible sparse row store, similar to Googles widely recognized bigtable project, and map/reduce processing for parallelizing data intensive workflows over a cluster.
Bigdata(R) comes packaged with a very high-performance RDF store supporting RDF(S) and OWL Lite inference. The Bigdata RDF Store is currently the only RDF database capable of operating distributed on a cluster with dynamic key-range partitioning of indices. The Bigdata RDF Store was designed specifically to meet requirements for very large scale semantic alignment and federation. RDF is a Semantic Web technology particularly well-suited to modeling graph-shaped data and metadata, such as an associative entity-link model, whereby actors are linked to one another in an ad-hoc fashion within the context of an evolving ontology of concepts for entity types and link types related to a particular problem domain. The Bigdata RDF Store is used operationally in data harvesting systems to create mash-ups of structured, semi-structured, and unstructured data from myriad sources in a schema-flexible manner.