The biggest challenge you face is a lack of patience and overabundance of determination. Your plan requires the complete defeat of a terrorist organization in 4 years. For perspective, the terrorist organization Al Qaeda, which is held responsible for the 9/11 attacks on America, still exists 15 years later.
The problem with this approach is that beating down terror is like playing whack-a-mole. It's almost impossible to take it all down without someone else picking up the torch. This is why we don't try to take on terrorist organizations in the way you want to .
Now I know this is a discussion of fictional worlds (no matter how politically charged the question's phrasing might be). However, since you chose to connect it to a real life conflict, I feel it is reasonable to seek expert opinion on the topic. In my opinion, the best analysis I have seen of ISIS appeared in the Atlatic last march: What ISIS Really Wants. It is a very lengthy and well written article, and near the end it starts to suggest a remarkably simple way to stop ISIS: hold still.
According to that article, the edicts upon which ISIS builds their movement upon are dependent on the growth of territory and the Jihadist waging of war against non-Muslims. Any group that declares themselves to be the caliphate not only must hold territory (as ISIS does now) but they must bring an offensive war against the non-Mulsims. Failure to do so simply proves they are not, indeed, the caliphate, and all of their claims are invalidated. Thus, if we simply deny them the capability to fight an offensive war, they will eventually consume themselves or lose their standing. It may not happen overnight. It may not happen in 4 years, but eventually the validity of ISIS as a caliphate will wane, and they will lose support.
Patience and self control are the undoing of ISIS. If you embody them, you can take them down, or change them into something which can exist on the world stage without all this violence.