@Rossimac
They wear at a slower rate to summer tyres if used in the correct conditions. You can get 4/5 seasons from a set of winter tyres in the UK and nowhere have I seen anything about them being 'done' at 4mm. They will wear out more if used as summer tyres.
Like any other tyre, they might have a little less grip as they wear, but that's all the negative info that I have found trawling around the web.
Amazing what one can learn spending 15/20 minutes on the internet.........