I'm looking at the differences now and it seems like it's mostly QoL changes: https://finalfantasy.fandom.com/wiki/Final_Fantasy_IV_version_differences#Super_Nintendo_Entertainment_System

In fact some here seems to indicate increased difficulty
"Excluding the Red Fang, all magical based items that could cast spells in battle were removed"
"The Alert item, the Summon Book as well as stat-boosting items (Silver Apples, Golden Apples, and Soma Drops) were removed, in addition to the Dark Matter"
"Cecil's Darkness ability is removed, in addition to all of the abilities that were removed in Easy Type (see above)."

Is this not a difficulty increase? Abiltiies have been useful being removed and items too.

It's commonly stated that FF4 US is easier than JP but I can't find any info actually validating this

I can't recall if there were any differences in enemy stats or experience gain, but the differences you listed do make the game a bit simpler.

In addition to what you listed, they removed references to hell, holy, and prayers in exchange for abyss, hades, white, and wishes. This leads me to believe they were trying to be more gentle with the US market. As I kid I always wondered why I was never able to use that cool sword technique that Dark Knight Cecil used in the paladin fight.

ig im not sure what your referring to? Are you playing the JP version? Cecils tech is darkness, there is no abyss, hades, white i assume would be holy, idk wtf wishes is, your comment doesnt add up.....

The original US release used all of those terms. I am aware that the sword tech is called Darkness, but I'm not sure it had a name in the original US release. I said that I wondered about it "as a kid" and, at the time, that's all I knew it as - a cool sword technique.

My understanding is that the OP is referring to the original US and JP releases in the early 90s. I am not aware that any other releases had any major gameplay differences.

gotcha, Ig i've totally forgotten names from the og snes version lol, i dont remember abyss or hades, i seem to remember white tho

FF2US is dumbed down compared to the original -- that's kind of the gist of the changes, they didn't think US audiences could easily understand the item spells and stuff like that -- but easier is kind of an eye of the beholder thing. The dolls fight, for instance, is much easier in FF4JP because of the items that were taken out of FF2US. But at the same time many of the FF4JP bosses have higher HP pools, some of the attacks have different formulas and work differently, and some of the scripts are slightly different.

As far as just the speedruns, complicating it is that FF4JP runners tend to mostly play glitchless categories, so it's harder to directly compare the runs themselves. But if you made me pick I'd say FF2US is easier mainly because the end game is a little less complex from a speedrun setting. CPU goes from 20K HP to 30K HP, the Rubi fight is a lot more of a slog, and so on.

There are not a lot of actual code differences between the original SFC and SNES releases. What differences there are I have mostly documented at https://ff4-documentation.readthedocs.io/en/latest/versions.html. Certainly nothing that directly affects difficulty like algorithm changes.

There are some (sometimes significant) changes to stats of the various monsters that you can look up at https://ff4kb.aexoden.com/info/monsters/. As Rivers mentioned, this especially affects the endgame. Some of the late game bosses and monsters had their agility and/or HP significantly reduced for FF2US (and some even more so for Easytype, which was a separate version released in Japan with all sorts of minor changes).

The removal of the abilities and battle items largely serves to make the game simpler, but more difficult (if you know how to utilize those abilities or items). It does however make inventory management much easier.

On balance, for the first time player, I'd say FF2US is somewhat easier, but nothing insurmountable. For more advanced players or speedrunners, the difference narrows but FF2US's moderate endgame advantages still make it a bit easier.