There are probably other ways to do it, but you could…
Turn off dynamic processing of url's since segment_3 will not be an article title. Then use {if}
statements. For example, something like:
{if segment_3 =="australia" AND segment_4 ==""}
<p>return this</p>
{if:else segment_3 =="australia" AND segment_4 =="melbourne"}
{embed="australia/cities"}
{/if}
Your template to process city information, would then be displayed on the country template. It's like setting up a single page site processor, within a template group. Basically, you are controlling the url with various {if} statements.
My only concern here would be about the potential number of cities you'd like to display. Manual specification of the cities pretty much means manual modification of template every time a new city is added. You might want something more flexible to add cities on the fly.