You error seems to be the usage of ' '. If your DB filed is numeric you should use the value without ' ' like in the next example:
SqlCommand cmd = new SqlCommand("select * from transactions where account_no=" + label4.Text, connect);
Better is to get your value from UI element (in your case label.Text), then use it in the SQlCommand:
SqlCommand cmd = new SqlCommand(string.Format("select * from transactions where account_no={0}", yourValue), connect);