VMS Help Sys Parameters, USER4  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    USER4 is a parameter that is reserved for definition at the
    user's site. The reserved longword is referenced by the symbol
    SGN$GL_USER4.

    On Alpha systems, this symbol is in the
    SYS$LOADABLE_IMAGES:SYS$BASE_IMAGE module.

    On VAX systems, the symbol is in the SYS$SYSTEM:SYS.STB module.