In this code Grid.Row="0"
or Grid.Column="0"
can be omitted, I wrote them just to understand how it is working.
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<GreenItem Grid.Row="0" Grid.Column="0"/>
<BlueItem Grid.Row="0" Grid.Column="1"/>
<RedItem Grid.Row="0" Grid.Column="2"/>
<OrangeItem Grid.Row="0" Grid.Column="3"/>
<YellowItem Grid.Row="1" Grid.ColumnSpan="4"/>
</Grid>