First of all, I want to thank you all again for this great discussion and your involvement.
I discussed the icon issue with the design team and we came up with the final decision to use the fa-sync-alt icon as its meaning is well established (as change) also outside the PF4.
Here it is:
I am aware that decisions like these are never going to be liked by everyone, but I hope it will nevertheless improve your experience just a bit.
Thank you