Steph_2002 - A trilogy by definition can only have three parts. A story in four parts I believe is called a quadrilogy.
By the way, I haven't seen Wonderboy or Tribute yet!!
My apologies my previous answer is of course the third part of a trilogy the fourth part being Mostly Harmless and the fifth part of a trilogy is The Salmon Of Doubt.
sft42, you were right the 1st time. Have the courage of your convictions!!
The 4th part of a trilogy is indeed 'So long and thanks for all the fish'.
*whispers behind hand* I think you forgot about 'Life, the Universe and Everything', book 3.