$ xsel -b | sed 's/^/    /' | xsel -b

# put two lines in the clipboard, "abc" and "123", for the example
# the n is a newline, and echo adds another newline to the end
$ echo $'abcn123' | xsel -b

$ xsel -b | sed 's/^/    /'  # output written to the terminal
    abc
    123
$ xsel -b | sed 's/^/    /' | xsel -b  # again, to the clipboard

#!/bin/bash
xsel -b | sed 's/^/    /' | xsel -b

$ chmod +x indent4

# test it:
$ echo $'abcn123' | xsel -b  # load clipboard
$ ./indent4                   # assuming it's in the current directory
$ xsel -b                     # show clipboard
    abc
    123