[Taha Okla]

هكذا :

كود :

   Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
       TextBox1.MaxLength = 4
   End Sub

   Private Sub TextBox1_KeyPress(sender As Object, e As KeyPressEventArgs) Handles TextBox1.KeyPress
       If Char.IsDigit(e.KeyChar) = False And Char.IsLetter(e.KeyChar) = False And e.KeyChar <> ControlChars.Back Then
           e.Handled = True
       End If
   End Sub