Since you have 4 days and are starting from Portland, the idea of going on the Olympic Peninsula is a good one.
It sounds like from Alan that only a couple of waterfalls are open. That could change between now and June, and if you have never been there, then even spending 3 or 4 hour there to shoot a couple of waterfalls might be worth it. But then I can see why maybe it would be best to skip that for now. Alan lives very close to that area, so hopefully he can periodically keep us updated.
You can just head west from Portland to Cannon Beach, shoot it and then head north on the 101 to Forks and shoot 2nd Beach, Ruby is a great beach too, and I like the idea of taking a Ferry across Puget Sound. You can certainly spend a day in the Fishermans Warf area, shoot the Space Needle if you want some city photography.
An other option would be to still add in Mt St Helens at the beginning, either backtracking back from Cannon Beach to head up there, and then swinging west to the Olympic Pennisula. Mt Rainier is an area I truly love, and that mountain is so majestic, but early June it still may have too much snow and I am not sure how much lodging is open up there that early. But it would be worth looking into also for one of your days.
It will be a fun trip and it will be interesting to see how you choose to fill up each day.