I love Vol 4. Don't get me wrong, it is different, but it's great in its own right.
It's not as old school and noir as the original run, it's a much crisper and "modern" style animation. I actually prefer most of the redesigns, but I definitely prefer the original run's darker, more Fleischer-esque animation and adult tone. That's not to say that it doesn't have its fair share of classy, dark, and mature themes, like Old Wounds, Growing Pains, Over the Edge. Plus some pure fun like Holiday Knights and Beware the Creeper.
Also, it really shows the progression of Timm & Co into the more expanded DC Universe, of course with the crossovers w/Superman, the appearance of Etrigan, etc. You can tell that it's slowly headed towards Justice League, which for me, isn't necessarily the best thing, for me at least.
I love Justice League, but it's just never really been my thing. I'm not one for the epic, cosmic stories. I much prefer the isolated, more grounded, more personal Batman world.