I've just noticed that the freezing also occurs when sitting at the WoW login screen, so I doubt addons have anything to do with it.

I also have exactly the same addons shared between all four clients (using symlinks/junctions) and have copied the exact same global settings to each client.

To further clarify, that fourth client window only seems to freeze when I move my mouse back over to my main screen, and will only unfreeze again when I move the pointer back over the frozen window.