Having been there for over a year I never saw the 400s...
That's because they don't exist, the webmaster of that site is just having a little fun with everyone.
de Havilland did investigate the possibility of providing a retractable nosewheel on the DHC-6 in the early 1980s, however, by the time all the work was done, sales had dropped off to the point where it was not really economically justifiable to go ahead with all the re-tooling that would be needed.
A retractable nosewheel would have provided a worthwhile increase in both cruise speed and range, in addition to providing better single engine performance under marginal conditions.
They never did pursue the idea of retractable main gear for the aircraft - it just didn't make sense. There is no-where within the existing fuselage that the gear could be retracted into, which means some kind of external fairing for the gear would be needed - kind of like an ATR. In addition, retractable gear would not be as strong as fixed gear - not a big concern for the nosewheel, but a major concern for the main gear, considering that the aircraft is primarily used on unimproved surfaces.
Michael