Re: X-MEN 4 release?
We seem to be forgetting some very important factors between the movies here. These are Marvel films and while I think that it's obviously no two big films will be in the same year, obviously the reason why Spiderman and X-Men have never been released in the same year.
X-Men July 14, 2000
Spiderman May 3, 2002
X-Men 2 May 2, 2003
Spiderman 2 June 30, 2004
X-Men 3 May 26, 2006
Spiderman 3 May 4, 2007
See the pattern? X-Men came first and gets three years between films, Spidey gets two, however They didn't want Spidey 3 and X3 in the same year so Spidey took the back seat till the next year because 1) X-Men came out first and 2) four years between a sequel seemed too much, so Spidey could wait 3. Now take a look at what we know and can speculate for upcoming releases.
Incredible Hulk (which is a reboot by the way) 2008
Wolverine/ maybe 2008 to 2009
Magneto/ most likely within a year from Wolvie so 2009 to 2010
Spiderman 4/ 2009? Maybe, but did we forget......
Captain America/ 2009 or 2010?
So hmm, that is an issue, while we don't know the success of Ironman or Cap yet, I think it's safe to assume that those two movies are probably going to be the next big hits, why? Well Ironman is Marvel's first "movie" so to speak as far say and production goes if I'm not mistaken. Also, most big hit comic franchises within Marvel release in around May, look above, with minor exceptions. Ironman is coming in May, and we all can assume that Captain America will definitely be huge. Also note that if an Avengers film is ever to be successful, Ironman and Cap both have to be pretty marketable as well as successful in order for it to work. So as far as an X4 release date. Well it can't and most likely won't be in the same year as a Spidey, Ironman, or Cap sequel. So when then? Hell there isn't much room.
As much as I'd love for X-Men to continue, the franchise has taken a huge loss of story with the deaths of three major characters. I'd say wait for Spidey, and most of Ironman and Cap sequels to pass then reboot X-Men. It will be quite some time and space for when that will roll around.