The "200" bit is a reference to the diameter when new (200 mm). Tormek lists the T4 specs as "Ø 200 × 40 mm (7.87 × 1.57")". It appears your stone has been well used 
I do not know if it is too worn to use, but I am sure someone else on this forum will chime in and enlighten us.
As to the choice of stones, I too used to sharpen my (and friends') knifes on Japanese waterstones. Last year I got a bit more time on my hands and ventured into the Tormek land. I am very happy with the system, the Tormek community and the results.
Most of my knifes are multilayered Japanese knifes, and just for the fun of it, I also purchased a SJ200 stone (I am on a T4), and that gives me a nice and shiny bevel - do I need it? No, the SG stone is more than enough, but I like it
I will second the suggestion above that you use the SG stone (or a replacement if it is too worn down) for a few months and see if that is sufficient for your use.
Welcome aboard!
I do not know if it is too worn to use, but I am sure someone else on this forum will chime in and enlighten us.As to the choice of stones, I too used to sharpen my (and friends') knifes on Japanese waterstones. Last year I got a bit more time on my hands and ventured into the Tormek land. I am very happy with the system, the Tormek community and the results.
Most of my knifes are multilayered Japanese knifes, and just for the fun of it, I also purchased a SJ200 stone (I am on a T4), and that gives me a nice and shiny bevel - do I need it? No, the SG stone is more than enough, but I like it
I will second the suggestion above that you use the SG stone (or a replacement if it is too worn down) for a few months and see if that is sufficient for your use.
Welcome aboard!