ABET is a 4 letter word that starts with A.

1. \A*bet"\, v. t. [imp. & p. p. {Abetted}; p. pr. & vb. n.
   {Abetting}.] [OF. abeter; a (L. ad) + beter to bait (as a
   bear), fr. Icel. beita to set dogs on, to feed, originally,
   to cause to bite, fr. Icel. b[=i]ta to bite, hence to bait,
   to incite. See {Bait}, {Bet}.]
   1. To instigate or encourage by aid or countenance; -- used
      in a bad sense of persons and acts; as, to abet an
      ill-doer; to abet one in his wicked courses; to abet vice;
      to abet an insurrection. ``The whole tribe abets the
      villany.'' --South.
            Would not the fool abet the stealth, Who rashly thus
            exposed his wealth?                   --Gay.
   2. To support, uphold, or aid; to maintain; -- in a good
      sense. [Obs.].
            Our duty is urged, and our confidence abetted.
                                                  --Jer. Taylor.
   3. (Law) To contribute, as an assistant or instigator, to the
      commission of an offense.
   Syn: To incite; instigate; set on; egg on; foment; advocate;
        countenance; encourage; second; uphold; aid; assist;
        support; sustain; back; connive at.
   

2. \A*bet"\, n. [OF. abet, fr. abeter.]
   Act of abetting; aid. [Obs.] --Chaucer.