Steam version of FF4 3D remake has normal and hard difficulty to choose, DS original is ONLY hard so get PC port instead of emulating. 2D version of Final Fantasy 4 was heavily cut down in American SNES release, it's missing half the skills, but was made easier to compensate. The music also sounds really thin and sprites are small as they developed the game for 8-bit consoles. So stay away from original release of 4, compared to, say, 6 where original version is already good enough.
So for 4: ignore SNES, choose any later version depending on which you think looks better, but be wary of DS difficulty. 3D version on Steam is very good and can let you change from normal to hard on New Game+ after getting OP armors and it removes touchscreen gimmick summon and changes a few quest to accomodate. Highly reccomended!