We usually write RnRS. If you're into Unicode or markup languages like TeX and HTML, you can write R⁴RS, R⁵RS, R⁶RS, R⁷RS, or generally RⁿRS.
And yes, I think that "The Revised Revised Revised Revised Revised Revised Revised Report on the Algorithmic Language Scheme" is pushing the joke rather too far, but R⁷RS is what name got consensus. It's modeled on "The {,Revised|Modified} Report on the Algorithmic Language Algol 60", where the usual abbreviations were R, RR, MR, but I like to write R⁰RA60, R¹A60, R²RA60 instead. Ditto with R⁰RA68, R¹A68 for the distinct language Algol 68. Finally, the incomplete report on the Scheme offshoot Kernel is R⁻¹RK.
And yes, I think that "The Revised Revised Revised Revised Revised Revised Revised Report on the Algorithmic Language Scheme" is pushing the joke rather too far, but R⁷RS is what name got consensus. It's modeled on "The {,Revised|Modified} Report on the Algorithmic Language Algol 60", where the usual abbreviations were R, RR, MR, but I like to write R⁰RA60, R¹A60, R²RA60 instead. Ditto with R⁰RA68, R¹A68 for the distinct language Algol 68. Finally, the incomplete report on the Scheme offshoot Kernel is R⁻¹RK.