Date format issue

Try this (using US locale (the date from which you're converting) (the last argument of the Table.TransformColumnTypes) to change both kinds of date successfully):

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMrDQNTDUNTIwMlHSAXEMjZA4BpbIHDMIJ1YHpEvfyEgfLqdvZIaTYw7hxMYCAA==", BinaryEncoding.Base64), Compression.Deflate))),
    ChangedType = Table.TransformColumnTypes(Source,{{"Column1", type date}, {"Column2", type date}, {"Column3", type date}, {"Column4", type date}},"en-US")
in
    ChangedType

This:

becomes this:

(I'm in the UK so these dates are showing as dmy, but they're proper dates so will show properly in your locale, that is, they're all August dates)