I wish we would get off this "Singer expressed interest in a Superman Returns" sequel. We all know why one didn't happen. It's because freaking Warner Bros didn't want one.
We know that Singer has a different relationship at Fox now than he did before. The fact that he served as Executive Producer and was responsible for the story of "First Class" would indicate that he is still interested at some level with directing a movie. Not to mention the fact that he attempted to get out of his iron clad Warner Bros directing contact for "Jack the Giant Killer" so he could direct First Class. "Superman Returns" was a totally and utterly different situation. One that needs to stop being brought up.
Calm down dear. It was a throwaway remark but a legitimate one in the context of you raising Singer's desire to direct an X4 as support for the contention that there will be an X4. FWIW, I'm a fan of Singer and of Returns but the point is that what he wants and what a studio wants aren't always the same thing.
I've also said before that I don't think there'll be an X4 by dint of the fact that too long has elapsed since X3, that 3 central characters died or appeared to die in that movie, that Stewart and McKellen may be too old to return (as indeed may Halle and Famke, in the bizarre world of Hollywood logic). We've had rebooted Hulk, Batman, Punisher and Spider-man within similar or shorter time spans.
If FC is successful enough to get sequels, I don't see Fox confusing fans by releasing alternate period sequels to it along with modern day X4, 5 and 6. I think that's about as realistic as the demands of those fans who want to see sequels to 24th century Star Trek alongside sequels to the JJ Abrams NuTrek. On the other hand, if FC isn't successful enough for a sequel (though I think it will be), then I think if there's another X-Men movie, it will be an out and out reboot. Given the success of Nolan's Batman movies and what will probably be a successful Spider-man reboot, there's no way Fox will do anything else.