I consider this to be the #1 Final Fantasy IV site.  It has Amano art, custom GIFs, armor guide, beastiary, character info, character skills, dropped items list, Game Genie cheat codes, tips, item guide, level listing, magic list, monster attacks, side quests, status guide, story summary, town/shop guide, full walkthrough, weapon guide, and a location guide.  There's even a comedy portion with several sections containing lots of hilarious FF4 stuff.  ^_^

  
 
 

Final Fantasy 4 @ Paladin.5u.com
This is another excellent FF4 site.  It has game info, FAQs, official art, lots o' screen shots, item/weapon/armor/magic/shop lists, box scans, FF Chronicles info & FMV screenies, cheat codes, and world maps.

  
 
 

Final Fantasy 4 @ Waterbearer.net
Character info, magic lists, some screen shots, Amano art, and some more stuff.

  
 
 

Final Fantasy IV Maps
This guy mapped out by hand the major dungeons of the game on graphing paper, listing treasures and all.  They're actually really good maps.  Much better than other FF4 maps I've come across made from emulator screen shots.