What is the process for expunging a 4th degree assault conviction in WA state

In most states your probation department will do it for free; call and see if thee is a similar provision in WA law. If not a private attorney should be willing to do it for $1,000 to $2,000.

You need to do a motion to vacate under RCW 9.96. Vacating an assault 4 requires the following: (1) no pending charges or convictions; (2) offense was not violent as defined by RCW 9.94A.030 (assault 4 is not violent); (3) offense was not DUI or related; (4) offense was not sex offense; (5) all probationary conditions were satisfied; (6) you have never had another conviction vacated; (7) you have not had a no contact order, protection order, or restraining order against you in the last 5 years, and (8) at least 3 years have elapsed since the completion of your sentence, including any period of probation (5 years if this was a domestic violence assault conviction.

If you believe that you satisfy all of these requirements, you can file the motion yourself or hire an attorney local to the court that you were convicted in. If you are successful, the conviction transforms into a dismissal and, by law, you can legally say that you have never been convicted.

Hope this helps.