Under what condition is a driver required to use Snow Chains (Schneeketten) on heavy trucks in Switzerland?

1. Whenever it rains in summer
2. Snow chains are strictly prohibited on all Swiss roads
3. Snow chains must be mounted on all wheels including trailer wheels at all times
4. When road sign 2.48 'Snow chains mandatory' (Schneeketten vorgeschrieben) is displayed; chains must be mounted on at least two driven wheels on the drive axle