It's roughy 8" x 4" x 5" and red in color.
From using it, it seems to be close to a polish stone but still has bite, so I'm guessing 4000? Someone help me out, it's a very nice stone.
You can assume it's 4K all you want, but assumptions are just that.
There just aren't that many stones anywhere near that size which get into this country in the ordinary chain of commerce. I know you work in a Japanese restaurant, so it may be dumb to assume that the stone was purchased through a North American retailer.... but -- speaking of assumptions -- you have to start somewhere. Based on your description of size, color and edge feel, my guess would be the (Giant) King 1200#. But there's no way to know without a picture AND accurate measurements. See if you can't upload the picture from your phone to your computer and post it here afterwards.
While you occasionally run into the perfect stone for a given knife and given purpose or its position within a larger set of stones, there's no really such thing as a perfect grit. Further, it's a mistake to think that one stone will be much like another because they're nominally the same grit. On the contrary, they can be different speeds, have different types of mud, different mud breakdown, and leave different polishes.
FWIW, Jon Broida probably knows as much about this subject as anyone in America. If you can afford the not inconsiderable freight, you might want to consider a couple or three of his Gesshin stones. They are the best on the market.