Adding options in comboBox and handle them

I have created a program with 4 Checkboxes and 2 indexes for the comboBox 1)Recommended , 2)Custom.Its easy to add in recommended when selected to check Checkbox1 and Checkbox4 but if i manually uncheck-check them the comboBox it does not automatically turn to recommended option. Fearther the checkboxes are in a groupbox. How is it possible to automatically turn the combobox index to "0" or recommended option ONLY if the checkboxes 1 and 4 are checked even if it has hapenned manually the same for Custom index if they are not checked

public partial class Form1 : Form
    {
     public Form1()
            {
                InitializeComponent();
                comboBox1.Items.Add("recommended");
                comboBox1.Items.Add("custom");

            }
            private void checkBox1_CheckedChanged(object sender, EventArgs e)
            {
                if (checkBox1.Checked == true)
                {
                     checkBox2.Checked = false;
                     checkBox3.Checked = false;
                }
            }

            private void checkBox2_CheckedChanged(object sender, EventArgs e)
            {
                if (checkBox2.Checked == true)
                {
                    checkBox1.Checked = false;
                    checkBox3.Checked = false;
                }
            }
            private void checkBox3_CheckedChanged(object sender, EventArgs e)
            {
                if (checkBox3.Checked == true)
                {
                    checkBox1.Checked = false;
                    checkBox2.Checked = false;
                }
            }
            private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
            {
                if(comboBox1.SelectedIndex == 0)
                {
                    checkBox1.Checked = true;
                    checkBox4.Checked = true;
                }
            }

            private void checkBox4_CheckedChanged(object sender, EventArgs e)
            {
            }


        }