isDate function

I want to validate the year function...if i want to validate the date enter by a user i will use isDate funtion but how if only year..
this is my script........


If Target.Address = "$J$28" Then
    If Sheet1.Range("J28").Value <> Empty Then
        If Right(Sheet1.Range("J28"), 4) <= 1999 Then
                    MsgBox "Tahun taksiran mesti >= 2000", , "Borang CP204"
                    Sheet1.Range("J28") = ""
                    Sheet1.Range("J28").Activate
             
       
        ElseIf Len(Sheet1.Range("J28")) = 4 Then
            Sheet1.Range("J28") = Mid((Sheet1.Range("J28")), 1, 4)
        ElseIf Len(Sheet1.Range("J28")) < 4 Then
            MsgBox "Tahun yang dimasukkan tidak sah", , "Borang CP204"
            Sheet1.Range("J28").Activate
            Sheet1.Range("J28") = ""
               
        ElseIf Len(Sheet1.Range("J28")) = 4 Then
            If Not isDate(Sheet1.Range("J28")) Then
                MsgBox "Tahun yang dimasukkan tidak sah", , "Borang CP204"
                Sheet1.Range("J28").Activate
                Sheet1.Range("J28") = ""
    Else
        TarikhBayar = Sheet1.Range("J28")
            If Sheet1.Range("L32") <> "" Then
                If IsDate(Sheet1.Range("L32")) Then
                    TempohHingga = Sheet1.Range("L32")
                    If TarikhBayar > TempohHingga Then
                        MsgBox "Tempoh awal perakaunan mesti < dari tempoh akhir perakaunan", , "Borang CP204"
                        Sheet1.Range("J28") = ""
                        Sheet1.Range("J28").Activate
                    End If
                End If
            End If

               
        End If
    End If
End If
End If


can u modify this code?i need a code that can validate the year by not using the isDate....is there any function to check the year...