Their won't be a Modern Warfare 4, as Infinity Ward said MW3 is the last of Modern Warfare series. I am sure Infinity Ward will come up with something all new though probably stay in modern to semi-futuristic era.
Again probably not going to happen. People are sick of Modern Warfare games it is done to death by everyone... and semi-futuristic, 3arc has that now so IW are not going to try and steal that just like 3arc never stepped into the Modern day era.
Yeah, MW4 would be a ridiculous marketing choice. And 3arc has done some modern stuff, because they ported MW1 and MW3 over to the Wii version. And they had to do quite a bit of stuff with that. I trust Treyarch with no matter what they do.