From the QnA on Bungie.net (the website of the games' creators)
Q Is this the last ever Halo game?
A It is the conclusion to this story arc.
Thats an ambiguous answer that doesn't confirm Halo 3 will be the final game, so Halo 4 is definately not written off.
To be honest the way I interpret that is Halo 1-3 form the first chapter in the Halo story, and potential follow up games will carry that story on through additional chapters or, as they put it, "story arcs" - and with Halo being the most successful game on Xbox, plus the movie coming next year, it wouldn't make sense for them to end it.
Still, you probably have a good two years before a 4th game, if one is ever to be made, will be announced - which is plenty of time to both get a strong community going, plus grab an alternative halo 4 domain.
It is a .net though, which will affect its value, and its a little lengthy - but with development or a targetted end-user, you're probably looking at mid-high XX
Just my opinion