I've tried contacting ASUS support, but they're unresponsive (I understand their customer support is notoriously awful, so I'm not hugely surprised by this). On the other hand, I could just order a 2x4GB set and return it if it doesn't work. It's Amazon, who are excellent on the returns/service front. I'll continue to try to get word from ASUS or a mobo-specific forum to find out if I can do anything to get 4 sticks running, but Googling about suggests that all the boards in the P7H55-M series have issues with 4 sticks (despite what the official specs might claim).
Thanks for all your help and advice; it's been a rather useful learning experience for me, above all else. I appreciate it.