As others have stated, the Dot 3 and 4 can be mixed, but I would suggest that if your current fluid is more than a couple of years old that you flush the system (brake and clutch) and refill with Dot 4.  This is one of those things that most of us don't do as often as we should.  Brake fluid is very hygroscopic, and water - in addition to vaporizing during braking and causing fade - also pools at low spots like wheel cylinders, calipers and slave cylinders and leads to corrosion.  Not all that expensive for a critical safety system.

Keith Hughes
'86 Westy GenV (Marvin)

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 Date:    Fri, 4 May 2018
 12:27:29 -0400
 From:    "ddbjorkman@verizon.net"
 <ddbjorkman@VERIZON.NET>
 Subject: Brake fluid
 
 Can one mix Dot 3 and Dot 4 brake fluid
 in a Vanagon brake system, or is that not recommended?
 
 Dave B.