i cant input more than 4 values in a tsxtbox, why?

Question

<div >

<table class="color">

<tr>

<td>

<asp:Label ID="lblUserName" runat="server" Text="UserName"

style="color: #333333; "></asp:Label>

</td>

<td>

<asp:TextBox ID="textUserName" runat="server" Width="192px" CssClass="background"

MaxLength="4"></asp:TextBox>

</td>

<td>

<asp:RequiredFieldValidator ID="rfvUserName" runat="server" ControlToValidate ="textUserName" ErrorMessage="*Enter User Name" ForeColor="Red">

</asp:RequiredFieldValidator>

</td>

</tr>

<tr>

<td>

<asp:Label ID="lblPassword" runat="server" Text="Password"

style="color: #333333; "></asp:Label>

</td>

<td>

<asp:TextBox ID="TxtPassword" runat="server" Width="192px" CssClass="background"></asp:TextBox>

</td>

<td>

<asp:RequiredFieldValidator ID="rfvPassword" runat="server" ControlToValidate ="TxtPassword" ErrorMessage="*Enter Password" ForeColor="Red">

</asp:RequiredFieldValidator>

</td>

</tr>

<tr>

<td colspan="2" align="center">

<asp:Button ID="btnLogin" runat="server" Text="Login" CssClass="Button"

Height="34px" Width="122px" onclick="btnLogin_Click"/>

</td>

</tr>

</table>

<asp:Label ID="Label1" runat="server" ForeColor="red" Text="" ></asp:Label>

</div>

this is my code and when i run and type username it takes only 4 values where i have to put values of 5-8