WPF - GridSplitter with three columns

I have an app with grid with 3 columns. The grid splitter between the first and second columns works just fine. To get the splitter over to be between the second and third columns I made a column for the splitter. (So now the the third column is really the fourth.)

When I resize the other columns also shrink. I assume that is because I have them set to be relative sized. But I don't know how to fix it.

Here is a XAML Pad Ready example of my issue. Plug this into XAML pad and then try to resize the last column to be smaller.

<Page
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="1*" />
            <ColumnDefinition Width="1*" />
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="1*" />
        </Grid.ColumnDefinitions>
        <StackPanel Background="#feca00" Grid.Column="0">
            <TextBlock FontSize="35" Foreground="#58290A"
                   TextWrapping="Wrap">Left Hand Side</TextBlock>
        </StackPanel>
        <GridSplitter Width="10" />
        <Border CornerRadius="10" BorderBrush="#58290A"
              BorderThickness="5" Grid.Column="1">
            <TextBlock FontSize="25" Margin="20" Foreground="#FECA00"
                   TextWrapping="Wrap">Right Hand Side</TextBlock>
        </Border>
        <GridSplitter Grid.Column="2" HorizontalAlignment="Right"  VerticalAlignment="Stretch" Width="5"></GridSplitter>
        <TabControl Grid.Column="3" Name="tabControl1">
            <TabItem Header="Add Links" Name="tabAddLinks">
                <Grid></Grid>
            </TabItem>
        </TabControl>
    </Grid>
</Page> 

Thanks for the help!