How to use Drush's concurrent execution capability?
This worked for me:

drush @site1,@site2,@site3,@site4 cc all --concurrency=4


I'm not sure how concurrent it actually was; the last message about site1 came immediately after the first message for site2, and all other messages were printed sequentially.  I did not measure to what extent each cc operation happened concurrently, or to what extent the system might have just been cpu or i/o bound, but it did seem to be nominally working.