What will happen to my bf on a WA state assault 4 DV charge?

Any number of things may happen — a lot will depend on what type, if any, of criminal history he has on his record. Unfortunately, once an arrest is made and the Prosecutors decide to prosecute, there will be a mandatory court appearance unless he reaches some sort of agreement with the Prosecutors. The case is now in the hands of the State — by that I mean it will be the State vs. your boyfriend, not you versus your boyfriend. The State does not really need you to testify but they can force you to testify through their subpoena power if you refuse, although this does not happen very often — it all depends on who the individual Prosecutor is on the case.

Hope this helps