radDropDownList with telerik in win forms

As per your code the code is as follows

private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
        {
            if (comboBox1.SelectedItem == "disable textbox 1")
                textBox1.Enabled = false;
            if (comboBox1.SelectedItem == "disable textbox 2")
                textBox2.Enabled = false;
            if (comboBox1.SelectedItem == "disable textbox 3")
                textBox3.Enabled = false;
            if (comboBox1.SelectedItem == "disable textbox 4")
                textBox4.Enabled = false;
        }

But you assigned the text as follows

this.radDropDownList1.DropDownAnimationEnabled = true;
            radListDataItem1.Text = "disable textbox1";
            radListDataItem1.TextWrap = true;
            radListDataItem2.Text = "disable text box2";
            radListDataItem2.TextWrap = true;
            radListDataItem3.Text = "disable text box3";
            radListDataItem3.TextWrap = true;
            radListDataItem4.Text = "disable text box4";
            radListDataItem4.TextWrap = true;

So your code should be

private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
        {
            if (comboBox1.SelectedItem == "disable text box1")
                textBox1.Enabled = false;
            if (comboBox1.SelectedItem == "disable text box2")
                textBox2.Enabled = false;
            if (comboBox1.SelectedItem == "disable text box3")
                textBox3.Enabled = false;
            if (comboBox1.SelectedItem == "disable text box4")
                textBox4.Enabled = false;
        }

Try this

radComboBox1.SelectedItem = "disable textbox 1"; 

Can you check the properties after  comboBox1.SelectedItem. // What are all coming here

Probably you are missing this