Square-Enix has recently released a spin-off/sequel to FFIV for mobile phones in Japan, and it may have a possibility of coming to WiiWare:

We just remade it, so it’s a bit of a stretch… [Laughs] Currently the sequel, FINAL FANTASY IV the After: Tsuki no Kikan is available for mobile phones in Japan. We’ll do our best to make this game available for our overseas audiences as well!

From this, RPGsite claims that after pursuing Square-Enix for answers, their sources discovered the strong possibility of a Wiiware release. 

Final Fantasy IV: The After centers on the son of Cecil and Rosa, two of the main characters from the original title, and uses a modified engine of the original SNES version of IV. The game is delivered in episodic format on Phones in Japan (as it is expected to be for any potential Wiiware release), and has already reached 1 million downloads.

I haven’t played much of FFIV on the DS, but I know it’s one of the more beloved games in the series, so hopefully we’ll see a US release on the Wii at some point.

Link