The most likely candidate is Gungnir; it’s the most recent release, and is closer to a traditional SRPG than the other possibles: Growlanser (for PSP), and Growlanser IV: Over Reloaded (the PSP port of Growlanser IV: Wayfarer of the Time).*
I’m partial to Growlanser (due in no small part to playing it for hours in a Japanese hotel at the end of my study abroad year). It kicks off the series, following the adventures of Carmaine and friends as they work for the king of Rolandia, and deal with war, peace, and (of course) an ancient evil.
Can’t Go Back is my favorite of the several boss themes from Growlanser.
Update: Atlus has revealed the fourth clue: the game is the fourth release in its series. Which, of course, doesn’t narrow it down as much as you might think. Growlanser IV is the obvious choice, but Gungnir (while Episode IX of the Dept. Heaven series) is also the fourth game in its series. Well played, Atlus. Your clues are ambiguous and will only serve to stir up resentment among the fans of whichever series isn’t picked.
*The most optimistic people are pulling for multiple games; Gungnir and Growlanser, so that none of the fans are angry. I hope they’re right; if Atlus picks one franchise over the other, the fans are sure to lose their collective minds.