It's been more than ten years since PSIV came out. It was billed as the last adventure in the Phantasy Star series on the US box. But, Phantasy Star has remained fairly popular ever since. Do you think Sega will ever make another game in the classic series? Not a new PSOnline or PSUniverse - but something that is a clear continuation of the story and style in the Genesis games.
For my part, I doubt it... but you never know, one day they might surprise us and decide to bring the series back with a fifth installment.