The dropdown values are numeric values which represent the positions of the texts in the list. So you would use
Code:
Sub Dropdown4()
Select Case ActiveDocument.FormFields("Dropdown4").Dropdown.Value
Case 1
ActiveDocument.Bookmarks("RiskTData").Range.Font.Hidden = False
Case 2, 3
ActiveDocument.Bookmarks("RiskTData").Range.Font.Hidden = True
Case Else
End Select
End Sub