Season three's The Hunted was the first time I noticed. The more prominent lifeboats and other serfuce detail was really noticeable. I actually didn't find out until a few years ago that it was a different model. I just assumed it had been lit differently or something!
So when was the first appearance of the four-footer?
In the awesome episode the Defector, from season three.
WHile I love the 6 footer its with two huge caveats.
1. My reception off of air, made the ship loss any detail at all. Even it's lines lost a huge amount of their grace. But you could see finally in generations how beautiful that model was, but on typical tv didn't hapen.
2. The fact that the 4 footer was easier to use, allowed the fx to produce a lot more model shots, then the 6 footer would have allowed.
So for practical real world use back int eh 80's and early ninites its teh 4 footer all the way.
But if you had my i person looking at the 4 footer, and the 6 footer, the 6 footer is beautiful, its one of The enjoys of the blurays and the increase size of tv's today's that help show the detail that was always filmed, but not always seen by the viewer.
But you know what even if we had HD content and large sized tv, I think the fat that the fx people were able to provide so many more shots, and attempt much more complex shots, due to the ease of use of the smaller model (especially with the smaller area they had to film in, after the initial work ILM did), means I would still rather have had the 4 footer, uglier and all.