You need to set AutoPostBack of all dropdowns to true and need to add SelectedIndexChanged event for first three drop downs. On SelectedIndexChanged of first dropdown you need to set selected value of second and it will fire SelectedIndexChanged of second and there you will have code to set selected index of third and similarly you will do on wards.
In Html
<asp:DropDownList ID="dropdownlist1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="dropdownlist1_SelectedIndexChanged">
<asp:ListItem Value="1">1</asp:ListItem>
<asp:ListItem Value="2">2</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="dropdownlist2" runat="server" AutoPostBack="true" OnSelectedIndexChanged="dropdownlist2_SelectedIndexChanged">
<asp:ListItem Value="1">1</asp:ListItem>
<asp:ListItem Value="2">2</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="dropdownlist3" runat="server" AutoPostBack="true" OnSelectedIndexChanged="dropdownlist3_SelectedIndexChanged">
<asp:ListItem Value="1">1</asp:ListItem>
<asp:ListItem Value="2">2</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="dropdownlist4" runat="server" AutoPostBack="true" >
<asp:ListItem Value="1">1</asp:ListItem>
<asp:ListItem Value="2">2</asp:ListItem>
</asp:DropDownList>
1 2
In code behind
protected void dropdownlist1_SelectedIndexChanged(object sender, EventArgs e)
{
dropdownlist2.SelectedIndex = someIndexValue;
}
protected void dropdownlist2_SelectedIndexChanged(object sender, EventArgs e)
{
dropdownlist3.SelectedIndex = someIndexValue;
}
protected void dropdownlist3_SelectedIndexChanged(object sender, EventArgs e)
{
dropdownlist4.SelectedIndex = someIndexValue;
}