If you hard-coded the textbox in the views then you can just call them by their variable name and set the text value from one to the other.
oTextBox4.Text = oTextBox1.Text;
However, if you are doing this dynamically then you need to use the
FindControl(string controlid)
function.
TextBox
oTextBox1 = View1.FindControl("oTextBox1") as TextBox
oTextBox4 = View4.FindControl("oTextBox4") as TextBox;
if((oTextBox1 != null) && (oTextBox4 != null))
oTextBox4.Text = oTextBox1.Text;