Just a supossition:
Wolverine première in May of 2009 isn't as huge as Fox expected and in the next weeks the boxoffice continue bad, so they start thinking about doing X4, have a few reunions, and finally decide to do it. In that same year, they sign a new writer and plan the beggining of the production for the summer of next year, 2010, to a worldwide premiere in May of 2011.
this doesn't sound so impposible, don't you think? hehe
Next Big X-Men Crossover
* 2017- X-Men: Age of Apocalypse
Edit: Forget about 2017:
Official announcement -> X-MEN: APOCALYPSE - MAY 27, 2016!!!!