With Square Enix jumping on the Xbox Live Arcade bandwagon the other week and confirming Crystal Defenders and Yosumin on Microsoft's service speculation about Final Fantasy arriving on XBLA was a given.
However, this is more than speculation as 1Up dug up the fact that there was a "details" page attached to "finalfantasy4" on the Japanese Xbox website. Though there is no information whatsoever 1Up went on to speculate that it might be a port of the Japanese mobile game Final Fantasy IV: The After. It could of course also be a re-release on the original. Square Enix hasn't exactly shied away from re-releases in the past.