Even though FFV looks a lot like FFIV when you're playing it, the devs likely figured out better ways to do things by the time they made FFV.
Yeah, like job system, FF4 assign a job to a specific player, while in FF5 you may have all jobs on each character.
FFIV is a programming clusterfuck.
I'm not sure what you imply but FF5 is clustered too. The good things is that it uses master tables that links things together.
No, sorry, "clusterfuck" is a vulgar slang term that means "big mess." The data and assembly in FFIV often are not organized very well at all, and when I say "the devs likely figured out better ways to do things," I mean that they figured out more effective ways to code, not necessarily that they made more fun ways to play.
For example, monster image data is entirely different, apparently. In FFIV, each monster entry is four bytes:
0: Size Index - monsters' rectangular sizes are picked from a table and can be 2x2, 3x4, 4x4, 17x41, or whatever. But any entry that's over 7F gets an entirely separate set of rules and is used to indicate either a special size, defined by an entirely separate five-byte table, or a PC enemy, like when you fight Yang or Kain, for example.
1: Palette Index
2-3: Sprite Pointer
So, unfortunately, the monster image table in FFIV seems to work nothing like that of FFV, so I'm afraid knowledge of one won't illuminate much about the other in this case.