score:1
If you have four weeks, plan a week on each of the four majors (Kauai, Oahu, Maui, Hawai'i). Each has a different character and different aspects to check out. And each has both local side and touristy side, I prefer local style, staying in small towns away from the beach resorts and dining at the drive in.
Inter-island travel is pretty much by air. There has been talk of ferries, but nothing much seems to come of it. You can go by ferry from Maui to Molokai and Lanai, but that is about it.
Hawaii has high and shoulder seasons, no low season. Spring and fall are shoulder seasons, so you may find better hotel prices and possibly lower airfares. But really killer deal airfares are rare.
Unfortunately "best" is so subjective and what I might like having lived there will be different than what you might like as a casual visitor (example, would you order spam at breakfast? that's local style and something I enjoy).