I'll try to be brief. I shall refer you to this argument that seems more reliable than some materialistic explanation of national behavior as a predictor of wars, famines, and mass slaughters :
http://www.hawaii.edu/powerkills/welcome.html
I see Rudolph Rummel's theory of Democratic Peace as a partial explanation of why genocidal slaughters are likely to be less a part of the next 4T than they were in the previous 4T.
I hardly see a 4T as a certainty of genocide and catastrophic war. It's not because I desire no mass killing; it's because this time we have lessons from the last 4T. In a way, Adolf Hitler has achieved an infamous immortality in that anyone who resembles him in his despotism and bigotry will be compared to him. Such will not end when the last of his intended victims dies off; cultural and political leaders of the Boom and Thirteenth generations born after his demise desire that that horrible man remain in the human consciousness as someone to be kept from power. Josef Stalin and Mao Zedong serve much the same end in some other countries as well.
Xenakis holds that genocidal wars happen because 4Ts enforce a Malthusian dialectic that overpopulation leads to catastrophic wars waged with genocidal hatred. Such may have been so in the past -- but humanity has largely found political solutions to economic distress and the means and desire to prevent population explosions. Contraception, abortion, and (even if this applies to only one country -- China -- it applies to what otherwise might be one of the most dangerous countries if it had a rapidly-growing as well as gigantic population) greatly reduce the tolerability of war as a solution for diplomatic failures. Peasants who have five sons may have less objections to sending off three to battle with a high possibility that two or three die in battle; a farm family that has one child is less likely to look upon wartime military service of the one child as simply a good means of bringing in some added income in hard times.
Another contributor to genocidal wars has been pathological leaders -- tyrants. A tyrant full of hate and fear can impose mass killings because he has the will to order mass killings, the means of suppressing knowledge of the killings, and the means of implementing them. Pathological leaders of course make aggressive war and/or genocide a certainty when they wield absolute power and seek military solutions to economic failures or -- worse -- to sate their lust for glory.
Democracy implies that leaders lack the power to order massacres, purges, and persecutions. Democratic leaders are under more pressure to deliver results through such measures as tax reform or public works -- not by invading other countries and raiding them for wealth and slave labor. Democracies allow, unlike tyrannies, effective movements of pacifism and for ethnic, religious, and cultural tolerance. Wars between democracies are rare in part because people in democracies have the freedom to display empathy across national boundaries.
Some good things have happened in the previous 3T that are likely to make the coming 4T less dangerous: most obviously, some very nasty dictatorships have fallen to democracy -- Chile, Argentina, Brazil, South Africa, Indonesia, Poland, East Germany, Czechoslovakia, Hungary, Romania, Bulgaria, Yugoslavia, and Albania. Five former Soviet republics (Estonia, Latvia, Lithuania, Ukraine, and Georgia) are now democracies. Since the aftermath of the last 4T Germany, Italy, and Japan -- the mortal enemies of democracy under genocidal regimes during the last 4T -- are at least as worthy exponents of democracy as is America.
We enter the 4T with more widespread knowledge of history. An exact replay of the previous 4T, this time with nuclear weapons in the possession of several major and minor powers, is impossible. The leaders that we now have or are likely to have are less naive about human nature (with one prominent exception until at least January 2009 -- cheap shot to be expected from me) than the leaders of 1933-1939. Intelligence networks are already in place, and all have the means of figuring out whether someone offering "Peace in Our Time" deserves credibility. That could be more important than having weapons systems in place.