Are you saying that what I want to do is illegal? SE as in Square Enix? Does SE even know these pirates let alone care?
I'm not even sure what you want to do since you don't mention it anywhere in your post. My understanding is that you want to port, or "demake" FF4 for the NES, but using your own game engine. Tell me if I was wrong.
And yes this is not legal since you don't own the rights for FF4, and it's characters. However, homebrewing a NES game is totally legal, and this include the case of one whose game mechanics and story could be very close to FF4, but with your own world, characters, enemies, .... I would encourage you to go that route instead if you do not wish to receive a cease and desist letter.
I'd put it in some other words : If you made a very sucessful game, would you like anyone to be distributing random clones of questionable quality, obviously based on your creation, without your own agreement ? Most certainly not. Even though I personally think FF4DS belongs to that cathegory^^
And this site is not illegal.
PS : Also, additionally FF4 is a particularly bad choice if you were to demake a game.
First it's graphics use few of the SNES capabilities and are very remembering of the NES. Copy/paste and removing one or two colours is not a huge challenge.
Second, FF4 has already been remake or ported to different platform 6 times or so
Third, Square Enix is one of the companies which defends their copyrights the morst aggressively.
PPS : You should also take a look at the WonderSwan Colour version of the game. It is in some way even closer to the NES than the SNES, especially sound-wise.