the current marshal version is 4 (closes #23167)

Patch by Dmitry Kazakov.
diff --git a/Doc/library/marshal.rst b/Doc/library/marshal.rst
index 124eb61..af43944 100644
--- a/Doc/library/marshal.rst
+++ b/Doc/library/marshal.rst
@@ -106,7 +106,7 @@
    format, version 1 shares interned strings and version 2 uses a binary format
    for floating point numbers.
    Version 3 adds support for object instancing and recursion.
-   The current version is 3.
+   The current version is 4.
 
 
 .. rubric:: Footnotes