It's two years after the original Batman: Animated series ended, and these episodes were made to air alternately with the newer Superman series on KidsWB Saturday morning block, which was still relatively new at that time. What's most notable about these episodes is that the cartooning style and character designs have become pretty uniform and set to the style that they will have kept through Batman Beyond to the Justice League series.
The real shame about The New Batman Superman Adventures is that KidsWB basically stopped production after 24 episodes, so we don't get to see the new revamps as much as we'd like (ex. Riddler, unfortunately, only shows up in minor cameos). A host of new villains and femme fatales appear, including Calendar Girl, Roxy Rocket, and the (dreaded) Farmer Brown and his mutant farm animals. Etrigan and The Creeper make their first animated appearances, but only get an episode each (Etrigan was later fleshed out in Justice League). Most telling is that certain plot threads feel a little rushed, including the introduction of Tim Drake as the new Robin, the departure and return of Dick Grayson as Nightwing, and even a new twist involving Two-Face. There just weren't enough episodes to say everything the staff wanted to tell (and maybe they told one Joker story too many).
It's more about comic book fights and action now. Bruce Wayne's a little stiffer and darker, and the stories have lost some of that humanizing touch the best B:TAS episodes had (with some exceptions of course). Is the quality still there? Yes. This is still excellent entertainment for kids and adults alike, and new viewers may be surprised at how mature these episodes feel compared to the stuff on KidsWB these days.
Definitely pick up vol. 4. It's a nice supplement to the initial 3 volumes, and continues the animated legacy of Batman nicely.
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