let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyVorVAdMmcIYpgmWmFBsLAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t]),
    #"Replaced Value" = Table.ReplaceValue(Source,each [Column1],each if Text.Length([Column1]) > 4 then [Column1] else null,Replacer.ReplaceValue,{"Column1"})
in
    #"Replaced Value"

使用transform而不是replace的替代方法。没有更好

let Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyVorVAdMmcIYpgmWmFBsLAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t]),
Trim = Table.TransformColumns(Source,{{"Column1", each if Text.Length(Text.From(_)??"") <5 then null else _ }})
in  Trim