Sub Insert_Char()
Dim Code As String, Car As String
With Feuil1
Car = .Range("b1") 'Lettre C ou autre
derlig = .Range("a" & Rows.Count).End(xlUp).Row + 1
For i = 3 To derlig Step 2
Code = Left(.Cells(i - 1, 1), 4) & Car & Right(.Cells(i - 1, 1), 5)
.Cells(i, 1) = Code
Next i
End With
End Sub