Final Fantasy IV: After--Return to the Moon will be a free download. At least the prologue will be.
In an interview with Famitsu, the game's executive producer Takashi Tokita confirmed that the prologue to FFIV's sequel will be available for free on Japanese mobile carriers. He also confirmed that the game will feature 2D graphics, as well as the Active Time Battle system.
Speaking of returns, it seems that you will once again be able to play as Cecil, Rosa and Cid, along with new characters such as Theodore, Cecil's son.
Unfortunately the game is only available on Japanese mobile carriers, but hopefully we'll hear about a port of the game in the near future. Stay tuned for more details as they emerge.