Most of the trouble comes between the marsh cave (where almost every enemy can poison you without much trouble) and continues until you beat the Ice Caves (where you start to have access to better healing spells like Life and Soft and enemies lose the ability to either cripple or kill your characters with poison and petrify status on all of their attacks). After this point the game doesn’t reach the level of difficulty anymore and you’ll drift on through the water temple and into the Lefa Tower before you’ll feel any kind of burn.
Even by end game your characters will most likely be able to handle the problems thrown at you. It’s natural for your physical characters to be able to deal insane amounts of damage and I’m betting that you’ve probably given Masamune to one of your Wizards giving you a third fighter with magical abilities and your fourth character can spam the usable item attacks thus you can go at it much longer than before although the biggest weakness of your team is the lack of a second healer but the Protect Rings will make a lack of a healer a rare occurrence.
With version other than the NES characters will usually have better spell casting abilities (they’ll either increase the amount of MPs in each level or they’ll have only one pool of MP to draw from instead of using the D&D inspired magic levels so that you’ll be able to cast Cures 1, 2 and Pure i.e. antidote more often than before) and by end game your characters will have enough MP to cast the highest level spells without the need to really conserve for the boss fights. They also increased the level cap from 50 to 99 and lowered the amount of EXP required for each level as well and in some versions have better equipment than before but it’s the MP limit that’s made the most broken.
