Hi petermcclure! And welcome to the ANA Forums! And thank you for making your first post.
You are correct that specialists tend to favor the approach they do, whether it be surgery or radiation. I think you are smart to seek out a team that does a high volume of both. That way you probably will receive a more unbiased opinion.
In terms of Washington State . . . you can go to the ANA forum Home page (what I'm about to tell you won't work from this page), and type "Washington State" in the Search panel on the upper right side, and click on the magnifying glass. Then you will see all the posts forum members have made about Washington State. Some of the comments won't be helpful, but some of them might be. There are quite a few posts to read through. Hopefully, you will find some helpful ones from people living in your area and their experience.
Also, please do not feel pressured to make a quick decision. A 4mm acoustic neuroma should not grow quickly. It is better to take your time and choose the right specialist for you than to make a hasty decision and then regret it the rest of your life.
Best wishes on your journey! And keep us posted on your progress!
Don