I was reading the Namco originally intened the Xenosaga series to span over 6 games but it was cut short to 3. But IGN hada recent interview with Namco about about a 4th edition to the series. But I doubt they will follow through with it here's the interview from IGN:
Question: Any plans on bringing new Xeno series to PS3?
Namco Bandai: The answer for this question is related to earlier questions. There are no plans at this point but we might consider bringing new Xeno series to PS3 if there are huge fan requests and sales.
Question: If Xenosaga Episode III is the last game of the Xenosaga series no matter what, would that change based on fan requests and sales? Is there a chance that Namco Bandai will make more games based on the Xenosaga universe (RPG or not)?
Namco Bandai: Yes, the Xenosaga universe is a grand concept and the number of ways we can think of to expand on this is infinite. We look forward to seeing player's reactions to Xenosaga III, and if reactions are good—who knows!
And if they did follow through with episode IV it probably wouldn't come to Australia we only have Episode II over here