Sum n values in a table with List.Range

LukeReds 

Try this way: 

List.Range(Source[value],  [week]- 4, 4) 

Create a blank Query, go to the Advanced Editor, clear the existing code, and paste the codes give below and follow the steps.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Lc25DQAxCETRXogdcPmqBbn/NpZZyB5fSBNBQoPEJr0RpPAqG3zLnlYtz7S1F34O/97oUv2gc/WLvqsL5+HcB5a9lwXTjun3AQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [week = _t, value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"week", Int64.Type}, {"value", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each List.Range(Source[value],  [week]- 4, 4) )
in
    #"Added Custom"

List.Range works with an offset and a count:

List.Range(list as list, offset as number, optional count as nullable number) as list

thank you Fowmy, shkran lak