Final Fantasy IV (known as Final Fantasy II on the SNES) is the one where you play a dark knight called Cecil, roaming the kingdom of Baron in search of love, crystals and the truth. Like FFIII, the main thing that sets it apart from previous retellings of the story is the graphical treatment it has received on DS, although there are enough minor tweaks and changes to make it worth playing again if the previous versions have faded in your memory.
New features include a dungeon map that fills in on the lower screen as you explore. Completing every last nook and cranny of it earns you a bonus at the end. There's also the option to retain abilities from characters that have left your party by 'decanting' their essences. It means you won't have to endure the loss of a particularly cool or powerful move just because the story robbed you of a friend, and it's a nice way to add some extra customisation.
Auto battle mode is another DS exclusive, but it isn't developed anywhere near as well as it could have been. It gives you the option of having the AI take care of all the fighting, which would have been a brilliant thumb-saver given the daunting amount of level-boosting grinding that FFIV demands. However, you can only specify one type of attack, which the AI will simply repeat until the enemy dies, your party gets wiped out or you take back the controls and save the day.
Compared to the gambit system in Revenant Wings, this is a horribly limited feature. Your elite adventurers don't even have the common sense to use potions if they're low on HP, making it dangerous to take your eyes off the screen at any time during a fight. When the enemies start getting tough, the auto battle mode becomes a liability.
And the enemies get very tough, very fast. The difficulty level is extraordinary, and it's impossible to survive in a new area without spending a long time winning random encounters against creatures in the previous one. It's probably the hardest Final Fantasy ever, at least in terms of the experience you need before you can hope to beat even lowly monsters. Maybe they made it like this to compensate for the relative shortness of the main quest.
It's certainly a treat for FFIV veterans, as it encourages you to explore every dungeon dead-end and refine your battle skills to probe for weak spots in the bosses. The same things still happen in the same order as they did in the SNES original 15 years ago, as well as in the more recent remakes, but this DS version is the ultimate edition of a classic adventure.
The graphics are particularly excellent. It seems to be built on the same engine as FFIII, except even smoother and more detailed. Apart from one long CGI sequence at the start, all the cinematics are done using the in-game character models, and their mouths have a surprisingly effective animation during the occasional vocalised scenes.