I would try to add a another week and go Portland to Denver via the adventure cycling Transamerica or Lewis and CLark routes. You'll get Rocky Mountain National Park, Yellowstone and the Grand Tetons, as well as a bunch of other nice American West riding. West is best.

If you have to land in SF, train and bus travel are rather a pain, but you can use Southwest Airlines or Frontier Airlines for reasonable priced flying between many western cities, including SF, Denver, Portland, Seattle, LA, San Diego. Other airlines have higher fees to carry the bike.

Another suggestion would be a portion of the "Sierra Cascades" route or a portion of the "Pacific Coast"

You can buy maps here http://www.adventurecycling.org/routes/network.cfm (or just look at their routes for ideas / download their GPS tracks). In summer (as long as that is June or later) you should be ok with any of these routes except the Southern Tier, which will be too hot.

Another possibility - if you love mountains (I do) - just start in Denver and ride around in the mountains in Colorado - you can easily spend 4 weeks in the mountains in CO without repeating yourself. If this sounds appealing, let me know and I can throw out a route I did a few years ago, that was intended to do all paved crossings of the continental divide in CO.