I just downloaded B4 this morning, got it up and running and am quite pleased at the performance boost over b3. I am running a dual opteron 275 setup, win XP, 4gigs of ram with /3g switch and it is still slightly sluggish, but no more than C1pro or DPP really. B3 was unusable for me.
I could not get the web ftp upload to work... and it would not develop a batch of images that I had set up and tweaked until I shut down the program and restarted. It also had problems changing the default previews/info directory.
I am QUITE happy that task manager shows it is hitting all four processors in my system though, B3 did not do this.
I have been using DPP primarily, but I think I am about to switch over...
too bad the lightroom forums wont let me login... seems that every nickname in the world is taken