Why go forward with "X4"? Answer: Because there is still interest and we all know Bryan Singer is very interested in directing another X-Men film. He almost directed "First Class" and if he wasn't already iron clad locked for "Jack the Giant Killer" he would have.
As mentioned before some of the plot and character stuff from "X-Men Origins: Magneto" was incorporated into the script and story for "X-Men: First Class". In fact Singer based part of his treatment from that script which is why it was cancelled...because First Class pretty much made it irrelevant.
I'm not saying "X4" is certain, all I am saying is that it's a very distinct possibility and there are tons of story lines and characters they do. It's even very possible if Singer comes aboard he will ahem ignore "The Last Stand"