Why isn't X-Men 4 happening ? Easy. Because they'd have to bring the old cast back: Hugh Jackman, Patrick Stewart, Ian McKellen, Anna Paquin, Rebecca Romijn, Shawn Ashmore, Aaron Stanford, Ellen Page, etc. Problem is, most of the cast has been signed on for 3 films (X-Men, X2, X:TLS) save for Jackman who signed on for 4 films (including a spin off) and Tom Rothamn & Co. don't want to renegotiate their contracts because they'd ask for higher salaries and they don't want that. Jackman for instance was paid $20 millions for Wolverine now add all the other actors cachés ontop of that, it's an awful lot of money and as they do sequels, the actors can renegotiate their salaries. Fox has found a cheaper way to keep the X-Men franchise: doing spin offs were you only have one mutant star and doing a prequel with the young X-Men, you're introducing a new cast with young unknown actors that they won't pay that much.
This is why we're not getting an X-Men 4.