Question: How much damage would a space probe cause if it can get about 4 lightyears away in 10 years and doesn't have any brakes when it arrives? Could it cause a global extinction event on the planet it was sent to? Do actual concept probes think about this?

Answer: 
**Not enough to cause a global extinction.**

I could plug things into equations, but I'm lazy. Plugging [relativistic kinetic energy of a 100kg mass traveling at 0.4c](http://www.wolframalpha.com/input/?i=relativistic+kinetic+energy+of+a+100kg+mass+traveling+at+0.4c) gives $8 \times 10^{17}$ Joules. The page on the impact that is hypothesized to kill the dinosaurs, [the Chicxulub impact](https://en.wikipedia.org/wiki/Chicxulub_crater#Impact_specifics), says that the energy released in this event was $4\times 10^{23}$ Joules. 

A quick google search shows that the juno probe is about $3000$kg, or about thirty times heavier than the mass I plugged in. This gives that your probe would release 0.00006 times the amount of energy as in the Chicxulub impact. This is a lot, but not in the grand scheme of things!
Calculating the damage done due to an impact is an imprecise business, but we could use the [KT extinction event](https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event) (that finished off the dinosaurs) as a benchmark. This impact had [an energy of around $4.2\times 10^{23}$ joules](https://en.wikipedia.org/wiki/Chicxulub_crater#Impact_specifics). The question is then how fast our probe would have to be going to deliver this much energy.

For a relativistic projectile the total energy is given by:

$$ E^2 = \frac{m^2v^2c^2}{1 - v^2/c^2} + m^2c^4 $$

and the kinetic energy is just this energy minus the rest mass energy $mc^2$.

Let's assume our probe has a mass of a ton ($10^3$ kg), which is around the mass of the [Voyager probes](https://en.wikipedia.org/wiki/Voyager_program). In that case to match the energy of the Chicxulub impact would need a speed of around $0.99999998c$. You're suggesting a speed of $0.4c$, and at that speed the kinetic energy would be around $8\times 10^{18}$ joules, which is a factor 50000 lower than the Chicxulub impact energy. Even so I wouldn't like to be standing underneath it when it hit.

But there are a couple of points that need to be made.

Firstly space is big - really big - and it's mostly empty. The chance of a probe hitting anything is so ridiculously small that no-one is going to take it seriously.

Secondly NASA does care what happens to its probes and this is part of the mission planning. For example [the Cassini probe is going to be deliberately crashed into Saturn](https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#Spacecraft_disposal) where the heat of reentry will destroy it (and any Earth life forms that might be hiding on it).