Just copy the text and use `sed` on your clipboard to transform your text.

On macOS:

    pbpaste | sed 's/^.*$/    &/' | pbcopy

Perhaps it's more readable/writeable with Awk:

    pbpaste | awk '{print "    "$0}' | pbcopy

Although this also seems to do the same thing

    pbpaste | sed 's/^/    &/' | pbcopy