Steve, the owners manual says that the 4mm disc is to have 80 grit on one side and the 150 grit on the other side. The 3mm disc gets the two higher grits, one per side. I didn't know you could do this either, so I bought an extra 4mm and 3mm disc which are not really needed unless I want to use other grits. For highly polished blades, they recommend taking an old dress shirt, attatching it to the 3mm disc with spray adhesive, and using the green honing compound to get the blade extra sharp - perhaps even "scary" sharp
The 1200 grit is going to wear out the fastest for obvious reasons. As a result, if you order a system, I'd get a more 350/1200 than anything else to have as back up. Right now, I haven't changed the paper yet, and it's still going strong after a number of chisels and two plane blades.