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?
The shot of a Romulan Warbird approaching the Enterprise at the beginning of The Defector was the first time the 4 foot model was used.