- Will not be like 3rd edition: Simple, Paizo has that market in the bag. People don't play Pathfinder because there is nothing else to play. If they really wanted D&D's 3.5 then they would play it. People play Pathfinder because it gives them what they want.
- Will not be like 4th edition: Apparently if 4th edition was such a success then we wouldn't be having this conversation. If they wanted to create an edition that was like 4th edition then that would be a waste of time because they could easily take 4th edition and tweak it to what they think the majority wants. They wouldn't need a 5th edition to do that.
- Will be it's own thing: The only way Wizards is going to get people from Pathfinder and keep people that play 4th edition is to create a new game that hopefully both sets of people like. In my opinion this would be the best option.
With 4th edition not lasting as long as was expected and it essentially being one of the shortest editions, I really wouldn't see Wizards trying to mimic much from that edition. Like any new edition, there will be some things carried over that may have not been really defined by a particular edition. I could be completely wrong about this but this is how I feel about the whole thing.
I also think they will back up from the digital age a bit. DDI was very convenient but it wasn't very practical because you would have people who never needed to buy a book and could play D&D just by spending a few quid a month. It's one of those situations where it's great for the customer but bad for the business and when it becomes bad for the business it eventually becomes bad for the customer because it can't be sustained.