I just took some measurements using a metric combination square as a depth gage. The width of the 250mm grinding wheels is 50 mm at the edge. The indent on the outside (EZYlock side) is 4.5 mm, meaning the width of the wheel is 45.5 mm at the bore. The width of the 200 mm diameter wheels is 40 mm at the edge. The indent on the outside face is 2.5 mm, meaning the width of the wheel is 37.5 mm at the bore.
In order for the T4 wheel to tighten in a T8, it is necessary to add an extra Washer for stone, Here is a link to the part:
With the added washer, a 200 diameter T4 wheel will fit in a T8 as well as any 250 mm wheel which has worn down to 200 mm. In general, I think it makes sense to use 250 mm wheels with the T8. Occasionally, a wheel is only available in 200mm or a 200mm hollow grind is specified for a particular blade.