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: 
First of all, even without an artificial braking system, a spaceship can slow down using tidal forces of the planets/astronomical bodies it passes by, just as it can speed up using gravitational assist. So, it will not necessarily reach cataclysmic speeds simply by travelling for a long time through space.

Now, let's estimate the energy that will be released if the space probe *[New Horizons](https://en.wikipedia.org/wiki/New_Horizons)* crashed into a planet like earth. *New Horizon* has a gross weight of about 400 kg. Its last recorded speed (the world record currently) is 16.26 km/s = 16260 m/s. Let's assume it crashes with this speed. And all of its kinetic energy is converted into heat. The heat released will be about 53,000,000,000 J. A ton of TNT produces about 4.184 $\*$ $10^9$ J of energy. The energy released in the crash event is about 10 times this energy. That's about enough to blow a large enough tunnel in an average mountain. The planet won't even notice.
The space probe won't do any damage as it will be vaporized in the exosphere. When the probe enters the interplanetary medium at a distance of the order of 100 AU from the star, it will collide with ions at a density of the order of a 5 ions per $\text{cm}^3$. The collisions with protons happen at a speed of 0.4 c, therefore at an energy of the order of 100 MeV, so this is quite similar to being exposed to intense alpha radiation. The temperature of the heat shield will be raised to about 600 K. The heat shield may tolerate temperatures up to about 6000 K before it will vaporize. This temperature will be reached when the density is $10^4$ times larger, so 50,000 ions per $\text{cm}^3$. But at 1000 km altitude above Earth you'll typically already have $\sim 10^{6}$ atoms per $\text{cm}^3$.

So, as the probe approaches an Earth like planet, the temperature of its heat shield gradually rises due to the increasingly intense bombardment of the ions and atoms it scoops up. At 10,000 km the shield will be red hot, 0.02 seconds later when it is at 4000 km it will most likely have vaporized, exposing the interior to temperatures of the order of 6000 K. The entire probe then simply vaporizes completely before even reaching 1000 km distance.

The atoms the probe consist of will keep on moving into the atmosphere until they collide with atoms. Because of the disintegration, each atom is now "on its own" while in a solid only the atoms at the surface would suffer collisions. All the atoms will have collided with atoms from the upper atmosphere already at a height of a few hundred kilometers. In these high energy collisions, the atoms get ionized and will then experience the strong Lorentz force due to the Earth's magnetic field. These processes will cause the energy of the probe to get dispersed well above 100 km altitude.