Just received an email from KFC. They seemed to notice the last one didn't work:
We’re sorry if you had trouble using this voucher the first time ‘round. To make up for our boo boo, we’ve got another $4 voucher just for you.
Edit: I've done a dummy order, and it appears to work this time.
Second edit (conditions) :
Minimum spend of $5 applies. Offer valid until 24/05/19. Not everywhere and not forever. Not available in Queensland, Western Australia, and the Northern Territory.
where's the voucher? Seems like a targeted offer.