If there are rings that have samples and a ring that is infested, I believe something either in Halo 5 or SA something will happen so that my guess will be that Someone from the UNSC has to go to an area infested and from there bring the flood back into the game.
I thought I knew a lot about Halo from just playing the games, but when It came to reading the books, you see different perspectives on so many things. Quickly, I became a huge fan of the books and still today I re-read the best ones in my opinion.
I don't think that they'll take Forge out, a lot of people are against forge in H4 but it's really just something to get used to, it's very different compared to H3 or Reach, but it's different in a good way.
I've always been wondering what had happened with the flood, they don't explain why the flood is not there. I was looking forward when I got the game to encounter the flood and see how they would look. In my opinion, halo is not the same to me without the flood.