Transform a specific value in a column

I think you will need to use the Table.TransformRows function here. See this code:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCkvMKU01VNJRMtQzNlGK1YGKGAFFjPRMzBEixmA1ZqYIEROwGlMjhIgpWI0JkogZWMTUAiFiDtZljmSXBVjEGGhyLAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Topic = _t, Values = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Values", type number}}),
    ConditionalTransform = 
        Table.FromRecords(
            Table.TransformRows(
                #"Changed Type",
                (x) => Record.TransformFields(
                    x, {"Values", each if x[Topic] = "Value4" then x[Values] /100 else x[Values]}
                )
            )
        )
in
    ConditionalTransform

I am just dividing any record with Value4 in the Topic column by 100, otherwise I am not touching it.

 becomes this:

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.