It's called a focal reducer or speed booster, metabones makes many, however they come with caveats.
A teleconverter makes the image circle of a lens wider, so it can work on the same sensor/film size as the lens you attach it to was originally intended for. You can make the image circle smaller but it won't work anymore on the same kind of sensor/film the lens you use it with was originally designed for. So you can only use it with smaller sensor, e.g. full frame lenses on crop bodies.
However, it works, it makes the lens (when attached to that smaller crop sensor) effectively wider and faster, but it will only make it as wide as it used to be in the first place on the full frame sensor. However, it will be faster than it used to be on the full frame sensor.
Btw, a focal reducer is nothing but a teleconverter mounted in reverse. Just like a good teleconverter is expensive, you can expect a focal reducer to have the same (expensive) price.
Edit after your edit:
In the general case, you can't do what you want. What you put behind the lens can make the image circle larger or smaller (or achieve some other kind of effect), but it does nothing in terms of changing the lightrays captured by your lens, which is what you want to do. You'd need something in front of the lens. Such elements exist, for example the WCL-X100 Wide Conversion Lens for the Fuji X100 series of cameras, or the various offerings for smartphones, but these elements are lens-specific, not generic elements. I suspect that producing such an element for the Canon 16-35/F4 would be extremely difficult and expensive.
