I've just seen that Lisa Buckingham has written about the "big four" accountancy firms. In the policy report that I submitted to the FRC in May 2006(.pdf), I said:
the notion of a “Big Four” is a mirage; and ... the audit market is
competitive (regardless of concentration or switching rates). Despite this the paper
suggests broad institutional reforms that could make the market function more
smoothly: permit cross-ownership; reduce the geographical basis of regulation;
relocate the burden of large-scale risk; and pursue measures to get company
information into the public realm.
Don't forget, it wasn't long since people thought the Premiership had a "big four". Look how that's turned out...