What you may not know is Washington is on NEC 2020, which changes the rules for meter pans and disconnects.  **The industry has responded with affordable products** that largely comply.

Keep in mind that when I say "disconnect", I mean "main breaker".  It is simply not affordable to have a separate disconnect from a main breaker, so the main breaker is used as the disconnect. 

Even if they were separate, the main breaker would have to be immediately after the disconnect, because you can't have a long run of non-breaker-protected cable waiting for a nail hit. 

Prepare for the possibility that you bought the wrong meter base.  It may be more expensive to "double down" on the wrong meter base than to junk it and get the right one. 

You need disconnect switch(es) at the meter. 
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The most affordable way to do that is a unit called a "Meter-Main", which places both the meter and the main breaker(s) in a single unit.  

Depending on how you read it, Code says you need either 2 disconnects or even 1 master disconnect at the meter.  You need to talk to your AHJ (permit issuing authority), as 400A disconnects are not affordable, so most AHJs are authorizing dual 200A disconnects. 

**Really, the bottom-line is what the local amendments are and what AHJs are allowing, given that following the letter of NEC 2020 
NEC 2020 on 400A services means very pricey equipment right now**.  As such, this is a conversation that you *need to have* with your AHJ, because only they can tell you which affordable options (if any) are available to you. 

As for the equipment already installed, somebody specced that.  The person who did either knows something we do not... or they don't and should be liable for any mistakes. I would stop taking their advice unless it aligns with what your AHJ tells you. 

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