I definitely agree the fourth and fifth lose some of the momentum but a lot of great events still occur. I read that the fourth and fifth were originally going to be one book but GRR Martin didn't like how much detail he had to leave out so split it (among other reasons). That would explain the number of chapters which don't really progress the overall story arc. I'm hoping the sixth book comes on in the next few years but unlikely. My other worry is that he somewhat rushes the last two novels (even if unintentionally) so that the HBO series doesn't have to play catch up. I'd probably suggest reading a synopsis of the last two book releases if you never plan on reading. It may just be enough to pull you back on board for the next one.