The blocks are whatever size you want. My 48" one started with 6.5" blocks, which means I cut the ones for the HSTs 7.5". Could have been a shade less, but I wanted even half inches.
http://www.quiltfrog.com/fourhour.html
BTW, 4 hours is a very liberal estimate. Mine might have taken less time if I had used only two colors, but I used eight.
Stitchinbee, that's a helpful mockup. I wish I'd had it a few weeks ago.