I personally think SSB4 will be better than Brawl for a number of reasons:
1. Its online will undoubtedly be better. Brawl was Sakurai's first project that used online features and he openly admitted that he didn't know which end was up which is why it ended up taking up about 15% of the development time.Sakurai now has a lot more experience with online play and the Wii U will have a much more stable online infastructure to work with than the Wii.
2. There will be more focus on the multiplayer. According to Sakurai, it's estimated that nearly 70% of Brawl's development was focused on the SSE and that seems to be one of Sakurai's biggest regrets with Brawl because, in the end, he wasn't able to create the immersive one player that he envisioned despite how much time he poured into it. With that being said, he'll probably be taking Miyamoto's advice this time around and focusing more on the multiplayer.
3. Character development will involve more of the staff this time around. For the past three games, Sakurai has been one of the few people who has worked with the characters and been in charge of things like character balance. As someone already said, he's already said that it will be more of a team effort from now on.
4. Sakurai actually wants to work on SSB4. With Brawl, he really didn't enjoy the project until it was just about done in early 2008 and he was reflecting on all the people he had worked with and all the connections he had made. For Brawl, Sakurai was pretty much black mailed into and I think he was openly resentful of having to work on it. If you ask me, this kind of shows with some aspects in Brawl like tripping.
As for what SSB4 will do to Brawl, I don't think things will change too much unless SSB4 is overwhelmingly better/worse than Brawl. There will be people who will flock to SSB4 and there will be some who stick with Brawl/Melee. In other words, it'll pretty much be the same as it is for any sequel to a game that has at least some competitive merit to it.