Thanks for your help..but i still have some error when applying this formula in Power Query Editor
For case 1 - add a new column called FULLNAME2
= Table.AddColumn(#"Changed Type1", "FULLNAME2", each Text.Combine(List.RemoveLastN(Text.ToList([FULL_NAME]),4),"")&"XXXX") >> This formula works fine!
If I don't want to add a new column, but to "update" FULL_NAME instead (means directly apply the logic in FULL_NAME column itself), what's the formula? I have found something like Table.ReplaceValue but not sure how the correct syntax..