The C4 corpus is a joint initiave by the following partners

Austrian Academy Corpus (AAC)

Swiss Text Corpus (CHTK) / Schweizer Textkorpus

Korpus South Tyrol / Korpus Südtirol

Digital Dictionary of the german language (DWDS).

The Corpus consists of subcorpora of each party, with a distributed access i.e. the corpus is 'virtual' - only the corpus hits are pooled together.

Query interfaces can be found here:

http://chtk.unibas.ch/korpus-c4