http://news.bbc.co.uk/sport1/hi/football/teams/l/liverpool/8102625.stm
It is understood Pompey still owe Liverpool £7m from the deal that saw them buy striker Peter Crouch last summer, and that is believed to have helped finance Johnson's transfer.
and presumably never did pay for Crouch, so surely Liverpool can tell them to do one.
Also, 4 year contract , signed June 2009. So it has expired.