Goodbye to the roof fans on the Series 400 - they have been replaced by two dedicated gasper vents on either side of the instrument panel (for cooling the pilots), and also replaced by additional ducting to provide more defrost air to the right and left side windshields.
The primary purpose of the roof fans was, I believe, to defog the inside of the windshields of aircraft that were not equipped with heated windshields. A heated (glass) windshield was not offered as part of the de-ice package on the very first Twin Otters - instead, you got a tank of alcohol and a windshield washer system to spray the alcohol on the outside of the plastic windshield.
The alcohol system was a 1960s solution to windshield de-icing, however, a heated glass windshield was introduced in the early 1970s, and it did a far better job of keeping both the outside and inside of the windshield clear. This kind of made the roof fans redundant, although they were retained (I guess for pilot cooling purposes) through to the end of Series 300 production in 1988.
The two gasper vents (visible in the photo further up this page, just underneath the autofeather switches, outboard of each primary flight display) provide a far greater volume of air for pilot cooling than the fans ever did, and don't present the same occupational hazard (hitting your head on the fan) that the roof fans did.
Progress...
Michael