Excel - extraire année d'une cellule texte

Option Explicit 

Const nomF = "Feuil1" 
Const co As String = "A" 
Const lideb As Byte = 1 

Sub ExtraitAnnee() 
Dim a 
Dim c As String 
Dim ranga As Long 
Dim li As Long, lifin As Long 
' derniere ligne 
  lifin = Sheets(nomF).Cells(65536, co).End(xlUp).Row 
' on y va 
  With Sheets(nomF) 
    For li = lideb To lifin 
      c = .Cells(li, co).Value 
      For ranga = 1 To Len(c) - 3 
        a = Mid(c, ranga, 4) 
        If IsNumeric(a) Then 
          If a >= 1900 And a <= 2000 Then 
            .Cells(li, co).Offset(0, 1).Value = a 
          End If 
        End If 
      Next ranga 
    Next li 
  End With 
End Sub