After review of our 4G coverage, I was able to locate 4G in Seattle WA and Tacoma WA. We also have 4G coming soon to Olympia Wa and Spokane WA. I show Monroe WA is not within a 4G area but is very close to Seattle WA which could account for your phone switching from 4G to 3G.
I have listed a link to view our 4G information and coverage area. You can also sign up for email updates about 4G information.