ImageButton click works only on seconed click

Hi,
I have a basic imagebutton:
<asp:ImageButton ID="ImageButton4" runat="server" ForeColor="#990000" OnClick="ImageButton4_Click" ImageUrl="~/images/speakers.gif"/>


And code behind:

protected void ImageButton4_Click(object sender, ImageClickEventArgs e) { ImageButton4.Attributes.Add("onclick", "popUp('PlayList.aspx')"); }
on the first click- a postback occurs but nothing opens as it should,
only on the second postback  the popup window appears.
Any clue?

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