The Reward for Cooperation is four years.  Perhaps the most important reward here was that his sentence would be served concurrently with his prior sentence (see here).

See Wall St Jrl (AP), Lobbyist Abramoff Faces Sentencing

Neil Lewis, NYTimes, Abramoff Is Sentenced to 4 Years in Prison

Doug Berman, Sentencing Law & Policy, Jack Abramoff getting cooperator’s benefit

Del Quentin Wilber & Carrie Johnson, Abramoff Sentenced to 4 Years in Prison for Corruption

Mike Scarcella, The BLT: The Blog of the Legal Times, Abramoff Sentenced to 4 Years in Prison

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