The Free On-line Dictionary of Computing (30 December 2018):

R4RS

   A revision of R3RS, revised in R3.99RS.

   (ftp://altdorf.ai.mit.edu/).

   ["The Revised^4 Report on the Algorithmic Language Scheme", W.
   Clinger et al, MIT (Nov 1991)].

   (1994-10-28)

   [Later revisions?]