I've travelled to Seattle and Portland several times (business and pleasure) and there are so many great sights (and sites) that you really can't do it justice in 4-5 days -- maybe 4-5 WEEKS!!
Only thing I might add - if you're into aviation the Boeing Museum of Flight
Museum of Flight is really interesting. I've been several times and see new stuff each time.
Have a ball - great area of the country to visit!!!