I haven't played The 4 Heroes of Light, but the first Bravely Default didn't strike me as being particularly easy, by JRPG standards. The annoying thing about that game was the fact that you were forced to do the same bosses over and over again during the second half of the game. One of the very few examples in which a single design decision ruined a game for me. Difficulty up until that point was pretty much about right for a JRPG, not noticeably easier than Final Fantasy 5-9 or the The Legend of Heroes games, IMO.
The issue with JRPGs is that level tends to be disproportionally important to success in comparison to other factors (compare with DnD-based games, for example), which means there is often very little than can't be trivialized by simply grinding a lot. The good news is that you can usually hit the difficulty sweet spot by simply not going out of your way to grind excessively. In the better JRPGs you don't really need to go out of your way to avoid encounters and gimp yourself to hit that sweets spot either, but they are rarely exceptionally challenging even then.
Obviously, I am mostly talking about conventional JRPGs in the FF4 mold, not about weirder stuff like the Saga games, dungeon crawlers, or tactical RPGs like Fire Emblem. The only classic-style JRPG that strikes me as having above-average combat difficulty is Final Fantasy 4 itself, and it achieves that by simply having a very fine-tuned power curve and tight balance.
Returning to Bravely Default, I recently completed the demo of Bravely Default 2 on the Switch, and I liked it a lot. I might well end up getting it once it's out. The Job system is fantastic as usual (still the best of all the Final Fantasy character systems), the visuals are gorgeous, the exploration is actually pretty decent by JRPG standards, itemization is solid, and it has plenty of welcome QoL improvements on the classic formula. Perhaps most encouragingly, the bosses were surprisingly difficult, especially the sand worm boss. To be fair, the latter probably benefited from the demo's level cap, but still, great stuff.