You can't merge part of a field, but if you were to use
https://www.gmayor.com/email_merge_addin.html to merge to separate documents, you could use the following macro with it to get the last four digits of any 16 digit number
Code:
Sub LastFour(oDoc As Document)
Dim oNum As Range
Set oNum = ActiveDocument.Range
With oNum.Find
Do While .Execute(findText:="[0-9]{16}", MatchWildcards:=True)
oNum.Text = Right(oNum, 4)
oNum.Collapse 0
Loop
End With
lbl_Exit:
Set oNum = Nothing
Exit Sub
End Sub