Unable to check a checkbox in a datagridview?

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C#

C#4.0

DataGrid

I have the following code:

Private Sub holidaysaveButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles holidaysaveButton.Click
    For Each dr As DataGridViewRow In DataGridView1.Rows
      Dim cell1 As String
      Dim cell2 As Date
      Dim cell3 As String
      Dim cell4 As String = "Holiday"

      If dr.IsNewRow Then Exit For

      If dr.Cells(4).Value = "True" Then
        Dim str1 As String = dr.Cells(1).Value.ToString
        Dim str2 As String = dr.Cells(2).Value.ToString
        Dim str3 As String = dr.Cells(3).Value.ToString
        Dim str4 As String = dr.Cells(4).Value.ToString
      End If
      If dr.Cells(4).Value IsNot Nothing Then
        cell1 = dr.Cells(1).Value.ToString()
        cell2 = dr.Cells(2).Value.ToString()
        cell3 = dr.Cells(3).Value.ToString()
        Dim con As New SqlConnection
        Dim cmd As New SqlCommand
        Try
          con.ConnectionString = "Data Source=xxxxx;Initial Catalog=xxxxx;User ID=xxxxx;Password=xxxxx"
          con.Open()
          cmd.Connection = con
          cmd.CommandText = "INSERT INTO exceptions (employeenumber, exceptiondate, minutes, code) VALUES ('" & cell1 & "','" & cell2 & "','" & cell3 & "','" & cell4 & "')"
          cmd.ExecuteNonQuery()
          MessageBox.Show("Your data has been updated")
        Catch ex As Exception
          MessageBox.Show("Error while updating record on table..." & ex.Message, "Update Records")
        Finally
          con.Close()
        End Try
      End If
      DailySummaryReview.holidayaddButton.Enabled = False
      Me.Close()
    Next
  End Sub

and I want users to be able to click on the checkbox in field4 and add all of the data plus the word Holiday to my database when they click on the box in that field, but right now it's not allowing me to click on the checkbox, can someone please assist? Also, I did have this working for that field but it would only allow me to add the first line of data, what in my code would not allow for the additional lines to not add their data as well?
Thank you