Then probably skip the 4th and only go for 5 if you want a conclusion and really liked the first three. Having said that there are some good bits in 4. Meh just get one of "Trilogy in 5 parts" all in one books and make your own mind up.
Then get both the Dirk Gently books which are also excellent.
__________________
He who disagrees with me in private, call him a fool. He who disagrees with me in public, call him an ambulance.