What is the difference between -h, -r, -s, and -k, in the "shutdown now" command in single-user mode?
As stated in the shutdown manual page (man shutdownentered in a Terminal window):

 -h      The system is halted at the specified time.

 -k      Kick everybody off.  The -k option does not actually halt the
         system, but leaves the system multi-user with logins disabled
         (for all but super-user).

 -r      The system is rebooted at the specified time.

 -s      The system is put to sleep at the specified time.


So to shutdown the Mac the -h option is the proper one. -r restarts it and -s puts it to sleep. The -k option doesn't halt your system at all.