So if they do an X4, which movies do you think it will incorporate into it's backstory? I'm thinking the first 2 and FC would be the most likely, since they're the ones Singer was involved with. Either that or the first 3 and FC, I don't really foresee XO: Wolverine playing much of a role.
I think the general rule will be that they wont reference each other at all and will avoid using the same characters. The trickiest thing will be Moira - if Xavier is 'reborn' on Muir Island and they want to build on that then Moira needs to be older than she was in X3 unless they come up with some throwaway line about her looking fantastic for her age... Maybe Beast tried to use a variant on the 'cure' he came up with when Moira was pregnant with Proteus and now she ages slower. Whether she should be dating Banshee is another matter - he's no more than 10 years younger than her.
Personally, I want X4 to be about the Marauders Massacre without Professor X but then I want a second trilogy. I think it would be quite doable to make X5 at least partially on Muir Island with the injured characters from X4. Mixing in Shadowcat's limminent death with the mystery of Proteus with a similar tone to X1 might be fun.