f4-key doesn't work in mann versus machine.

Seriously. It just does stuff that seems built into my laptop. How do I fix it?

Since you mentioned a laptop, I assume it has a "function" button (Fn).

Sometimes you need to push that button and then one you plan to use simultaneously in order to get the alternate functions on your keyboard, and sometimes, the alternate functions are preset, and you have to press that button and the button you want to use simultaneously, so you get the "normal" function.

Anyways, try that. If it's not that, I'm all out of ideas.

I use a laptop for mvm as well. My f4 key is some sort of screen function, but if I hit shift+f4, it usually works. Perhaps try that combo?

You can also simply re-bind the key. I've shuffled a few keys around myself (moved the Use Item key to V, so I don't need to reach as much, for example).