aTV3 has never been jailbroken, so I doubt aTV4 will be. Apple figured out how to prevent it. I hope I'm wrong.
Actually, the ATV4 is a lot more likely to be jailbroken than the 3.
The ATV3 doesn't have any way to develop apps for it. The ATV4 does.
Once you have app development on the platform you have a lot of new ways to poke and prod and a lot more potential avenues to attack the system's security and jailbreak. ATV3 was mostly a closed box; heck the firmware would disable the USB port early in the boot process. ATV4 is a whole other world.