As we can read in Wikipedia, Final Fantasy 4 was developed and published by Squaresoft in 1991 as a part of the Final Fantasy series. The game was originally released for the Super Famicom in Japan, but has been ported with minor changes to the Super Nintendo in USA, re-titled as “Final Fantasy II” due to it being the second of the series released there.
After completing Final Fantasy III for the Famicom (NES) in 1990, Square planned to develop two Final Fantasy games—one for the Nintendo Famicom and the other for the forthcoming Super Famicom (SNES), to be known as Final Fantasy IV and Final Fantasy V respectively.
Due to financial and scheduling constraints, Square dropped plans for the Famicom game and continued development of the Super Famicom version, retitled Final Fantasy IV. A mock-up screenshot of the cancelled Final Fantasy 4 for the Famicom / NES was produced for a Japanese magazine.
In the released Final Fantasy 4, fans of the saga found some unused stuff still hidden in the game. One of the characters you get at the beginning of the game, Kain, has an unused spell casting animation. In the Slick Productions Forums you can read a theory about where his magic spell list is and see Kain in game using the spell.
Yang, another character that you get and lose and get again, during the battle against him, has an attack text-script, and that includes a script for a character in your party, but it seems that the developers “Dummied” it out. The same script has Yang attack Kain, but at this point, you don’t HAVE Kain in your party, hinting at a possible different story event. Another unseen event was hinted at in the code with a character named “Anna”, she has battle animations and everything but they were never used. More info can be found in here.
Also, there’s a picture of the back of FFIV’s japanese case, shows something that doesn’t happen in the game itself anymore. And another beta picture was found in Nintendo power. As Deathlike2 explained in the GameFAQS forum:
It looks like DK Cecil, Kain, and Yang with Young Rydia and Rosa. None of that is normally possible.
1) Fire2/Fira cannot be learned by Young Rydia until she comes back as Older Rydia.
2) Kain leaves right after the Mist event.
3) That enemy formation does exist, but not in a castle environment (the battle @ Fabul never has such a formation).