Have a look at the following link:
http://www.kenwoodworld.com/uk...
It says that the blender attachment fits all "Chef/Major models beginning with KM", including your KM400.
It's a bit heavy, but it gives great results and is very easy to clean. Despite its size, it is also suitable for small quantities.

Hello all
I received the Kennwood KAH358GL mixer today. It definitely does not fit on my KM400. The blender can no longer be screwed onto the appliance, the attachment has been changed.
Best regards.