The #4 movie - which movie series will have it first?
X-Men - Wolverine and First Class are prequels, so technically they aren't "X-Men 4"
Spider-Man - the fourth movie from Sony is a reboot
Blade - no news about Blade 4
The Avengers - too early to tell
Fantastic Four - the third movie from FOX will be a reboot
Iron Man - very possible unless Marvel Studios doesn't want to continue the Iron Man movie series after IR3.
I'm going with Iron Man.