Speaking to the New York Post, Piper explained that she did not want the series to outstay its welcome.
"I felt like in story terms it had run its natural course," she said. "Even though we could do the funny clients for years, there's only so much ground I think one could cover without it becoming a little bit dirty and seedy.
"So as much as it pained me to say goodbye to the show, I didn't want us to get tired or old or bad."
Piper explained that some fans are hoping for a movie adaptation of the show but added that she would be content if the programme ended with the final episode of the fourth series.
"I feel like it's where it should be, in terms of conclusion," she said. "It's been a really wonderful ride - at times, literally!"
The final series of Secret Diary of a Call Girl is currently airing in the US on Thursdays at 10.30pm on Showtime.
> Billie Piper 'struggling to find work'