lucenematchversion is a mandatory config field in solrconfig.xml in Solr 4.0. It used to be optional. While it is defined for the search core, it seems missing in the stats and OAI core.
As a result, it defaults to Lucene_24. e.g.
WARNING: the luceneMatchVersion is not specified, defaulting to LUCENE_24 emulation. You should at some point declare and reindex to at least 3.0, because 2.4 emulation is deprecated and will be removed in 4.0. This parameter will be mandatory in 4.0.
In attachment a logfile. Thanks to Ben Ryan for pointing me to the cause. Will issue a pull request for this soon.