I recommend everyone using either RowSimilarityJob or ItemSimilarityJob
or RecommenderJob (both of which use RowSimilarityJob internally) from
Mahout 0.4 to update to the latest trunk version of Mahout as
RowSimilarityJob has had a serious bug (see
https://issues.apache.org/jira/browse/MAHOUT-610 for details).
Sorry for the inconvenience.
--sebastian