Try this:-
ASPX
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
<asp:ListItem Enabled="true" Text="Select Value" Value="-1"></asp:ListItem>
<asp:ListItem Text="Value1" Value="1"></asp:ListItem>
<asp:ListItem Text="Value2" Value="2"></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList2_SelectedIndexChanged">
<asp:ListItem Enabled="true" Text="Select Value" Value="-1"></asp:ListItem>
<asp:ListItem Text="Value1" Value="1"></asp:ListItem>
<asp:ListItem Text="Value2" Value="2"></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownList3" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList3_SelectedIndexChanged">
<asp:ListItem Enabled="true" Text="Select Value" Value="-1"></asp:ListItem>
<asp:ListItem Text="Value1" Value="1"></asp:ListItem>
<asp:ListItem Text="Value2" Value="2"></asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownList4" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList4_SelectedIndexChanged">
<asp:ListItem Enabled="true" Text="Select Value" Value="-1"></asp:ListItem>
<asp:ListItem Text="Value1" Value="1"></asp:ListItem>
<asp:ListItem Text="Value2" Value="2"></asp:ListItem>
</asp:DropDownList>
ASPX CS
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
DropDownList1.Enabled = true;
DropDownList2.Enabled = false;
DropDownList3.Enabled = false;
DropDownList4.Enabled = false;
}
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList2.Enabled = true;
}
protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList3.Enabled = true;
}
protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList4.Enabled = true;
}