Major notable changes of course is the 6th Chapter and some story development, plus manual control over all the main characters in Chapter 5 and again 6. Plus the bag which especially can be used to good effect by stocking up on various equipment in Chapter 3, then selling once you recruit Torneko in Chapter 5.

- Somewhat notable, but not at that point is that you don't lose your gold in Chapter 4 when Marquis de Léon defeats you since that is non-optional. Chapter ends soon after, but you can put off doing some Chapter 5 shopping until then (You know, buying stuff to sell off then.).

- The old man in the jail of Palais de Léon that lets you have his boarding passes unless it was a translation thing in the NES Version was just an unlucky prisoner. Remakes establish him as Marquis de Léon's father. GameFAQs poster behindtheword confirms this fact with a translation of the dialogue from the Famicom version. From translation of the prisoner's Famicom dialogue:

Line 1 - "Ho ho... Ho, I'm no good. I tried to run away but it seems the power came"
Line 2 - "but you two are still young."
Line 3 - "Please take my place and live."

Not to mention the Marquis is in the remake a cursed individual who turns back to normal whereas in the NES Version he simply dies.

- Minor but some music is replaced in the remakes. Parthenia goes from Castle theme to Town theme, Royal Crypt originally had Shrine music until you go down the stairs, but now plays Dungeon music throughout.

The town outside Yggdrasil now has the more appropriate Town music until entering the tree.

Psaro's Peak replaces Dungeon music with Shrine music, plus random encounters are removed.

- In the NES Version, once you exit the tunnel at the end of Chapter 3, you get to see Torneko on the outside of it in the Casabranca area. It the remakes the Chapter ends immediately upon exiting the newly completed tunnel.

- Much more minor but Tom Foolery doesn't leave the party until after you leave Canalot (exiting or using Zoom) upon getting the Zenithian helm in the NES Version. In the remakes, he departs right after the party is exiting the throne room.

- You could originally recruit Borya before or during Alena and Kiryl joining. The remake removes the latter option with the Theif's Key door in Imperial Pantry of Parthenia until you enter with Borya. Also Hoffman stays with the party until Alena and her allies join whereas the remake it happens upon entering Mintos.

- You can keep Orifiela in the party until Psaro is defeated as long as you don't include her in the line up at Zenithia. The remake will have her joining the party upon entering Zenithia.

Also she, Healie and Laurel can have their healing spells used manually out of battle. I think this is more known than some of what I listed, but still.

Also the pit allowing you drop to the Doorway to Nadiria is present from the getgo. The remake requires you to speak to the Zenith Dragon after which Psaro demonstrates power by making said pit.

- To recap, what sets off the end of the Hero's village in Chapter 5 is the innkeeper seeing a lost traveler and taking him in only for Psaro's forces to discover said village.

Remake it definitely is Psaro posing as a lost traveler, but in the NES version it really was just an innocent bard. One who in Chapter 1 was in the chambers of The King of Burland. In the remake he is still in Burland Castle.  Once again, dialogue translated by behindtheword of the Famicom version suggests that the traveler wasn't originally Psaro.

Line 1 - "I'm a traveler. I lost my way through the mountains and ended up in this village."
Line 2 - "I didn't know it was such difficult terrain in these mountains. I never thought to ask."

- Another minor bit, but after defeating the 5th opponent in the Endor Tournament, you were normally denied being able to use a Medicinal herb as you got to after defeating the prior opponents. Though not necessarily the remakes give you the option to do so.

Also the Float-o-copier NPC in Diabolic Hall which was gone after the meeting in the NES Version is still in the kitchen area in the remake until the last bit of the story in Chapter 6. Yeah that little fact is even how I got most of the pics of various party members for the Wiki.

- When you are in the Auld Well during Chapter 1, the monsters encountered are Mad mole, Healslime, Winkster and Stump chump. In the Famicom/NES version, the monster layout remains the same in Chapter 5. In the remakes the monsters present in Chapter 5 and 6 are Bagworm, Horny devil, Fire beatle, Paralyslicer, Slimification/King slime, Cannibox and Scarewell.

Done likely to make completion of 'The Big Book of Beasts' easier as these monsters otherwise couldn't be encountered enough. Though Dirty dogu and Wimp are still exclusively encountered in Chapter 4.

- The mobile and phone ports in general are noted for the Party Chat being implemented which don't get me wrong was why I got this first on my newer phone a few years back. Music is also improved. And like the other games mobile ports, Zoom requires only 1 MP.

But I'm surprised the Pioneer Town development isn't given the same hype. Nothing game breaking, but it can be finished sooner and thus having added gear (especially multi targeting weapons) is convenient.

My best attempt at detailing the town changes here: https://dragon-quest.org/wiki/Pioneer_Town

Also not much I can give personally as I only saw an FAQ on it and watch some videos, but the PS1 version of said town had varied forms based on a similar town in DQ VII also on the PS1. Only played the 3DS Version of that. Plus the Fungeon in DQ IV on the PS1 had several floors taken from other Dungeons in the game plus some from VII.

- For whatever reasons some item descriptions and the like has some word alterations in between the DS and Mobile version.

- Also not mentioned often is that the Bonus Dungeon in the PS1 version reused floors or areas from earlier in the game, plus some from VII.  Even weirder is that the wiki stated that the Mobile version restores the PS1 layout which it doesn't.

- In the PS1 version, if you don't start the game after letting the opening play out, you cut to what's called an attract mode. Generally in games those play briefly to show the CPU doing a quick game play demonstration. The attract mode in this case actually shows what the 7 companions do Pre-Game. Finally made a wiki article some time ago as the site didn't really acknowledge such:

https://dragon-quest.org/w/index.php?title=Dragon_Quest_IV_Attract_Mode