1) a sentence concurrent with prior ones
2) a cooperation deal for a lesser sentence
3) a life sentence that's already been imposed
4) a sentence far less than you'd get post-trial
1) Manafort is cooperating, giving up Trump/someone else.
2) Trump illegally promised Manafort a pardon.
3) Mueller offered Manafort a long consecutive sentence—which Manafort took because he thinks if the facts come out at trial he'll get far worse.