What is the best method of checking for all possible combinations of values for some text boxes?

Let's say we have four textboxes.
<asp:TextBox ID="txtBox1" runat="server" text="1" />
<asp:TextBox ID="txtBox2" runat="server" text="2" />
<asp:TextBox ID="txtBox3" runat="server" text="3" />
<asp:TextBox ID="txtBox4" runat="server" text="4" />

What is the most effective way of getting values that show all possible combinations of the above if the control has a value and is not an empty string.
For example I use this and it works but gets large.
Dim strCase as string = string.empty
if txtBox1.text <> String.empty AndAlso txtBox2.text <> String.empty Andalso txtBox3.text <> String.empty AndAlso txtBox4.text <> string.empty then
strCase = "1234"
End If
if txtBox1.text <> String.empty AndAlso txtB ...