An Eternal Sabbath Keeping

Coming to a conclusion the author to the Hebrews writes in 4:9, "There remains therefore a Sabbath rest for the people of God." The introduction of a heretofore unknown word ("a Sabbath rest") which is different from every other reference to rest in Hebrews 3 & 4, is the focus of some attention. Is this being intentionally used? Is it revealing something distinctive about the aforementioned rest? These matters are considered along with the exhortation of 4:11, "Let us therefore be diligent to enter that rest..."