bug#50786: Did find more info https://unix.stackexchange.com/questions/190317/gnu-parted-resizepart-in-script

Shows it as undocumented command line?? option?
Also, figured it does need 3 dashes. Thought I might have 
mistyped it, and tried it with command and it failed. 
So does need 3 ---. Not sure why it is undocumented??

Note that 'isatty' test, which will fail. The 
'pretend_input_tty' is an undocumented command line 
option, which can be switched on via ---pretend-input-tty.

So, if you want to use parted from a script, my answer is 
the following:

/sbin/parted -a optimal /dev/loop1 ---pretend-input-tty 
resizepart 4 Yes 522239s