Peter:

I've found the four cordon, two level, system of little value. The bottom level lacks vigor and is shaded by the upper level so doesn't really contribute much to yield. For those reasons I've gone to a one level canopy. Depending on the trellis this could involve two, four, or rarely more canes.

Yield is determined by canopy area and light interception plus the number of bunches retained. I thin pretty heavily because I'd rather have a modest yield of very high quality fruit vs a huge yield of mediocre fruit.