Let me get it straight: the ending is very Adamsian. I have an unsubstantiated feeling (one could say "a belief") that Douglas Adams never cared much about the endings.
The reason I'm giving it my favorite 4.5 is the pure joy of Adams capturing so many concepts from what is by now computer science folklore, but at the time of writing the book was on the forefront of software engineering. The book is written in 1987, mind you.
And perhaps that's another reason many gave it three stars. Adams, not unlike his character, really liked to talk about computers in detail.