I'll just answer this myself.
Ver 4 installs and works fine. It is a new plugin from ver III, so there is complete backwards compatibility with old projects - this is great ;-)
Ver 4 isn't asking for any money and recognises my ver. III registration - so everything seems cool.
I'm really only interested in the reverb in ver 4 - and even that can be done with other plugins - but it's handy to have here.
EDIT - I also noticed that the presets seem to be interchangeable between ver III and ver 4. If I load a preset in ver 4, and add a reverb setting, and then save the preset - I can still load the preset in ver III!
This is really cool - and will really help with compatibility with old projects.