Originally, Phantasy Star IV was to have been designed for the Sega CD system, and would have been vastly different from its final incarnation. Features like 3-D dungeons (such as those featured in the first Phantasy Star), full motion video cutscenes, voice acting, and much more were planned. Unfortunately, poor sales and lack of support for the Sega CD platform caused a change of plans midway through development. Most of these features were scrapped from the final design, and the end result was a cartridge game that bore little resemblance to the original plans. [info from Wikipedia]
Some unused items and techniques can still be found in the game’s code: we can read more info about these at PS Algo!
Feeve was used during the debugging or the making of Phantasy Star IV, as a test to see if techniques worked properly. There is really no evidence to support this, save that the technique cannot be gained by playing the game normally.
The Blood Axe, the Spiral Slasher are strange items that were made inaccessible as part of normal game play, but never actually removed.
Two planet maps, one for Motavia and one for Dezoris, are coded into the game, and can be viewed using an edited saved game. Sega simply chose to not allow players to have access to the maps.
Thanks to Klinger Bea Arthur and Robert Seddon for the contributions!