The Halo series is a prequel to the Marathon series, I believe.
In Halo's universe, the year is 2552 when Halo: Combat Evolved starts.
While Marathon's story is set in 2794.
That's what I heard, at least. As some people claim the planet the Chief was heading towards on Legendary was Marathon.
But, to answer your question correctly, no. I don't think there will be a Halo 4. They've ended the trilogy. But, that doesn't mean they can't bring the same characters back and continue the story in a different way. I'm just sure they won't be calling it Halo 4.
Actually, that planet has to do with a planet mentioned in the Halo novels. I haven't read them myself, but my brother was telling me about this very thing not to long ago. Apparently there were four (or maybe it was five :x) spartans at first, and the other three (or four) are stuck in stasis on this planet.... or something generally along those lines. It'd probably make more sense if I'd take the time to read the novels, but I'm to lazy XD;
So I don't know, it could lead to another game, or just another novel. Or maybe even the Halo movie that was been started and stopped so many times >:( But I doubt it will be Halo 4, if it's another game at all it'll probably be a stand alone.