This is probably a partial duplicate of #5169 and the associated fixes mentioned there but it seems to be more too.

With 4.0.6-rc.0 I am unable to specify ajax url and minimum input length using data-* attributes.
This does not work: http://jsbin.com/lazabud/edit?html,output

It works when I move the options to the function call. I think #5169 covers the ajax url but not the minimum input length.

This still doesn't work properly: http://jsbin.com/vuleyoq/edit?html,output
It executes the query immediately before I enter any text.

This works correctly: http://jsbin.com/zupeki/edit?html,output

These all work with 4.0.5.