Alright, finished this one.
Overall, I liked it. It's not an amazing game by any means, and it overstays its welcome, but it's worth playing now that the performance and stability issues have been resolved. I liked it enough to push through the annoying puzzles and finish it.
As a followup to BT1-3, it both works and doesn't work.
In terms of story, yes, you get to run through and kill all the bosses from BT1-3 again ("for good") and then kill an even more awesome boss above them. You can also get the ultimate weapons from those games here. From a story point of view, it basically revives a "story" that was already finished, and then wraps it up with a SUPER BOSS above even the bosses from BT1-3. So I guess it works in some respect, since BT1-3 were just killing a super boss, then finding out there's an even more awesome evil boss in the next game, etc.
In terms of mechanics, it's not much of a followup to BT1-3 at all. It's a lot like M&MX in a way, as it has fixed encounters throughout the world but party building is heavily based on certain builds. With M&MX, this could leave you in a very bad situation indeed if you didn't pay much attention to how you built your party (even on Normal difficulty). In BT4, it doesn't matter at all because halfway through the game you have already developed superheroes (I used two practitioners, one rogue, two fighters, and a bard; replacing that bard with another rogue would probably have completely broken the game I think), and for the last 10-15 hours you are just assigning skill points at random because it doesn't matter if you choose to add another armor point (-1 physical damage) or another point of constitution (+1 HP).
I raged a lot about the puzzles earlier in this thread, and I still maintain that there are too many puzzles, but I'll admit that the final dungeon was a lot of fun. They finally (after 30+ hours) introduced some wrinkles into the moving stones and gear puzzles, and it felt nice to solve them. I still think that eliminating all the puzzles in the outdoor areas would have improved the game a lot. I don't think anyone wants to solve a dungeon full of puzzles, only to enter an outdoor area and then solve the exact same puzzles over again.
Character development is fun for the first 10-15 hours. At that point, earning even a single point of strength or an additional point of armor class really makes a difference, never mind reaching a capstone skill (i.e. advanced class) that gives you overpowered passive abilities. Compared to BT1-3, gaining a single level in BT4 (for the first 10-15 hours at least) is much more significant, and I think this is the single area where BT4 is clearly superior.
Graphically, the environments are very beautiful. Now that the performance has been fixed, it's a joy to explore. NPCs universally look horrible, while enemy models are generally fine. I had no qualms with the graphics.
Sound is fine. Party voices are uniformly awful, and I would suggest turning them off, but you can't (completely). The music is whatever. It's nice as background music. In contrast with BT1-3, bard songs are all played in battle so you won't be walking around listening to whatever song you bard is playing. In a way, this is the biggest difference from classic BT. In BT1-3, your bard plays a significant role as a buffer and his songs basically stay in effect until you play something else or reload your game or whatever. In BT4, your bard is basically no different from a "practitioner" (spellcaster) in that his songs only buff you within a single combat (having said that, bard songs differ from spells mechanically, and also are all buffs and do not do direct damage).
I'm glad I gave the game the second chance, and I would consider replaying it with the director's cut on a harder difficulty.