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If you make a 4 node blend material like so:
When you try to paint blend weights, the color you paint instantly jumps to the maximum value, and there's no way to paint it softly.
The vertex colors default to white, but if you vertex paint in " colors" mode to make them all black first, then you are able to paint softly.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-17079 in the post.