I had the exact same experience as the OP: I was locked out of my account due to multiple unsuccessful login attempts (not by me!); I called to unlock (which worked fine), and later I received an email saying that I had to do a PIN reset.

I don't know what's going on with IHG. I wonder if they had some sort of mass-scale breach attempt. It's hard to understand why IHG clings on so perilously to a four-digit PIN as the sole login method.