Double square brackets have a special meaning, and you can’t have 4 characters in an icon.
But you can prefix with text
to allow any characters. Maybe this is a compromise:
#popclip
name: Wikilink
icon: monospaced [[
javascript: popclip.pasteText('[[' + popclip.input.text + ']]')
description: Adds double square brackets to selection
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)
Edit: changed text
to monospaced
because it looks better in monospaced font IMO