You will have to expand one of your upper views to fill the remaining space by setting android:layout_weight="1" on it. This will push your last view down to the bottom.
Here is a brief sketch of what I mean:
<LinearLayout android:orientation="vertical">
<View/>
<View android:layout_weight="1"/>
<View/>
<View android:id="@+id/bottom"/>
</LinearLayout>
where each of the child view heights is "wrap_content" and everything else is "fill_parent".