2" is the main difference, the T4 uses an 8" wheel, the T8 uses a 10" one. There are speed differences between them as well. I gave up on my T7 sold it and bought the Sorby ProEdge instead, absolutely nothing wrong with Tormek, they are the masters of the wet grind system often imitated never equalled, but wet grinding takes time to set up, and clear up at the end of the day, I have no water in my workshop so had to remember to take a bottle of water down to it if I was going to sharpen, silly niggles really as the Tormek really does do a good job of sharpening a lot of things.