Hi,
It really is up to your plans and itinerary. If you can see Hiroshima and travel to Tokyo within 7 days, you would get great savings using a 7 day pass. The best route would be to avoid backtracking as much as possible, and put as much long distance travel within 7 days as you can.
But since you already plan to see Tokyo, you can largely skip Osaka during the day. Use the time for Kyoto/Nara, and at night you can enjoy many good nightspots in Osaka like Dotonbori and the Umeda Sky Bldg. Kyoto Station at night is worth exploring as well. Kobe has some good lookouts at night like Mt. Rokko.
You could do Hiroshima in a day if you start early in the morning - see the Peace Park/Museum, Shukkeien Garden and castle, but the real jewel of the region is Miyajima and is someplace you should not miss. You can see most of the best in ¾ of a day. For Yokohama, be sure to see the Chinatown as well as the Sankeien Garden.
Best of luck.