Try these two things out in the dingus. The first one has a string, that looks like a closing fence, indented four spaces. The second one is only indented three spaces.

 ~~~
    ~~~

 ~~~
   ~~~

The spec definitely wants the second one to be an empty code block, but it's a bit ambiguous about the first:

The closing code fence may be preceded by up to three spaces of indentation, and may be followed only by spaces or tabs, which are ignored.

Pulldown-cmark checks "spaces of indentation" relative to the indentation level of the opening fence, but the reference implementation, commonmark-hs and markdown-it all seem to check relative to the start of the line.

" ~~~#4\n    ~~~\n" -> [
  Start(CodeBlock(Fenced(Borrowed(""))))
  End(CodeBlock)
]

" ~~~#4\n    ~~~\n" -> [
  Start(CodeBlock(Fenced(Borrowed(""))))
    Text(Boxed(" ~~~"))
  End(CodeBlock)
]

" ~~~#4\n    ~~~\n" -> [
  Start(CodeBlock(Fenced(Borrowed(""))))
    Text(Boxed(" ~~~"))
  End(CodeBlock)
]

Pandoc {
    meta: {},
    blocks: [
        CodeBlock(
            Attr(
                "",
                [
                    "#4",
                ],
                [],
            ),
            "   ~~~",
        ),
    ],
}