Assuming Romero, Night of the Living Dead, style slow shuffling zombies have risen from the dead to consume the living, it would be numerically impossible for them to muster in sufficient numbers to allow them to take over the world. Bare with me now, assuming that every dead body buried within the last year, which is rough 64,579,200, were to rise on night one as zombies. Now that’s assuming none were cremated or were so mangled that it would make reanimation impossible, crushed in an auto accident for instance. This also assumes that the zombies rose in uniform numbers all around the world, which also isn’t realistic as mortality rates vary greatly from developed to developing nation. All that aside, we’re just taking of about 9.6 zombies for every 1,000 persons. Now assuming all the undead horde rose up at the same time at night and takes everyone by surprise. In the process let’s say on average each zombie killed two people or about 129,154,400, within the first 12 hours. By the second wave of deaths, following 12 hours, people are no longer being taken off guard and being to fight back, but since they don’t quite know how to kill them another two persons are killed per zombie.
At this point, roughly a day into the rising about 258,316,800 people have died, with no zombies causalities. By this point there’s still about 996 persons per zombie. Now not all survivors will want to or be capable of fighting back, so let’s just assume only one in 50 people wants to and is capable of fighting back, which is roughly 19 (rounding down) people per zombie.
In a worst case scenario let’s say that everyone who engaged the zombies died fighting and in the process eliminated all the zombies in the process. To makes things even more grave and realist, lets say another 21 persons per zombie were killed the subsequent fighting. That’s 44 killed per zombie or 2,841,84,800. Now though this is a huge number in a very worst case scenario, but considering that something like over 3.8 billion people survived its by no means a Day of the Dead style apocalypse. And yes I’m aware that some people would turn to zombies after being attacked, but as you can see from Dawn of the Dead that takes time and in the mean time all the original zombies would have been large eliminated and any new zombies would be killed either before they turned or in much smaller numbers by people who by this point would be quite good at eliminating them. A much more possible scenario for a level 4 outbreak leading to an end of the world would involved the infected instead of an undead rising.