Well, it certainly sounds as if Lupin's coming back in 5, so I'm a happy camper... I hope he survives, however; I've got serious doubts, since he's a fairly tragic character to begin with, and I rather suspect JKR may kill off a more significant character next time.
I think she's almost going to have to pull back a bit in 5, in terms of telling a smaller story that will still further and have an impact on the big story arc. The number of characters and subplots in 4 were almost overwhelming at times. Voldy (yes, I call him that, it's easier to type) is theoretically still getting his followers reorganized, and with 6 & 7 yet to come, I don't think he can pull anything too major yet in 5. He may be more of a "behind the scenes" presence next time, as we start to see the Death Eaters reorganize. I would also expect Snape to have a bigger role in the next book, since JKR's set him up on his Highly Mysterious Mission.
In which book are we finally going to get the sort of background on Lily that we got on James in Book 3? I think that's where some big twists are going to come to light.