Michael Ausiello is reporting that Oscar nominee, Emmy winner John Lithgow (3rd Rock from the Sun, Footloose) has signed on for season four of DEXTER. His role will be

Walter Simmons, an unassuming suburbanite who has been living a dual life as one
of America’s most prolific and deadliest serial killers. When the mad man (who
is dubbed the “Trinity Killer” because he always kills in threes) relocates to
Miami, Dexter becomes obsessed (or inspired?) by his efficient killing methods
and ability to evade capture for almost three decades.

Also of note is that Keith Carradine will be returning as F.B.I. Special Agent Frank Lundy.

Excellent news. Carradine had very fun chemistry with Jennifer Carpenter (Deb).