god's final message to his creation

officially, and finally, i have finished the 4th book in the trilogy of four - So Long, and Thanks for All the Fish. i can say that i was very pleased with all four of these books but would highly recommend to anyone considering reading the Hitchhiker's Guide To The Galaxy series that you read them all at once rather than take breaks in between books. the plots, characters and story lines scatter themselves all over the place and it seems (at least to me) to be a good idea to keep them fresh in your mind when trying to absorb just under 800 pages of information.

my one question, which is certainly not the ultimate question, is posed to anyone who may have read the trilogy of four only to realize that there is a fifth book in the trilogy of four. is it critical that i read Mostly Harmless with the others? i got quite a sense of... conclusion i s'pose the word could be.. from So Long... and didn't see the need to continue the series. the first three books definitely let you think you needed to read more, but i found that i feel i know the whole story. well, i know what should have been the whole story except i KNOW that there's more because it was on the shelf beside the other four books.

so do i spend $5 on a copy of Mostly Harmless to have a real sense of finality in this series or will it just stir up more of the unknown and make me wish there was one last book to make it a trilogy x 2?