Following in the footsteps of games like Mortal Kombat, Gears of War 4 has received the R18 rating in Australia, the highest rating available, citing primarily the "high impact violence" as the reason.
Interestingly, the beta was only rated MA15, suggesting that perhaps the final product is even more violent than what we've seen so far.
I doubt anyone is surprised by this classification, but if you want an idea of the cause for this rating, watch the video below.