Source : Free On-Line Dictionary of Computing

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?]