If I remember correctly..
The large black one is your diagnostic port
The small black one to the right of the picture is for your cigarette lighter illumination
The white one is to power your cigarette lighter
The one at the top is for your heated rear screen switch
And I don't see a 'fourth' one? Unless that red thing is another plug? In which case, it probably isn't used.. although I think red plugs were used on the airbags.
I'm not 100% sure though and it's 10 to 2 in the morning so don't fancy going outside to rip my dash apart..