List

Private Sub Worksheet_Change(ByVal Target As Range)

Dim oldValue As String
' The previous value.
Dim newValue As String
' The new value.
Dim updateValue As String
' The potentially new value, subject to tests.

Application.EnableEvents = True
On Error GoTo Exitsub

If Target.Column = 4 Then
    If Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Then
        GoTo Exitsub
    Else: If Target.Value = "" Then GoTo Exitsub Else
        Application.EnableEvents = False
        updateValue = Target.Value
        Application.Undo
        oldValue = Target.Value
            If oldValue = "" Then
                updateValue = newValue
                Target.Value = newValue
            Else
                If InStr(1, oldValue, newValue, vbTextCompare) Then
                    Target.Value = Trim(Replace(oldValue, newValue, ""))
                    newValue = updateValue
                    Target.Value = newValue
            Else:
                Target.Value = oldValue & ", " & newValue
            End If
        End If
    End If
End If

Application.EnableEvents = True
Exitsub:
Application.EnableEvents = True