Based on the answer in this forum, just add the FlipView.ItemsPanel inside the FlipView and the problem will solve.
<FlipView Name="mainPage">
<FlipView.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal" />
</ItemsPanelTemplate>
</FlipView.ItemsPanel>
<WebView Name="wv1" Source="http://www.google.com/" Width="1024" Height="768" />
<WebView Name="wv2" Source="http://www.google.co.id/" Width="1024" Height="768" />
<WebView Name="wv3" Source="http://www.google.co.uk/" Width="1024" Height="768" />
<WebView Name="wv4" Source="http://www.google.com.sg/" Width="1024" Height="768" />
</FlipView>