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!
Let's assume it travels at a constant speed of .4c (since we don't know anything about its acceleration) and its mass is 100kg. At those speeds, the relativistic effects are minor compared to the fact that I just totally made the mass up out of thin air, so we can do a classical solution.

Using $E=1/2mv^2$ we find that the energy is $1/2 \times 100 \times 119916983^2 = 7.19004143 \times 10^{17} ~\mathrm J$. Using my favorite page on the internet, [Orders of Magnitude (Energy)](https://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)), we see that that's on par with detonating the Tsar Bomba, the largest nuclear weapon ever built. It's 6 orders of magnitude off from the energy released by the Chicxulub meteor event that is believed to have caused the extinction of the dinosaurs.

Thus there's little risk of an extinction event. That and hitting a planet by accident 4 light years away without trying is pretty hard!