It's true there was fog in Kos which was limiting visibility to 4 km,
The international definition of fog is a visibility of less than 1 kilometre (3,300 ft); mist is a visibility of between 1 kilometre (0.62 mi) and 2 kilometres (1.2 mi) and haze from 2 kilometres (1.2 mi) to 5 kilometres (3.1 mi). Fog and mist are generally assumed to be composed principally of water droplets, haze and smoke can be of smaller particle size... (Wiki)
but as far as I remember IFR flights do not have a minima for visibility.
It depends on the type of arrival approach available and the certification of the aircraft/crew.