If's not a fallacy. When you go from 2 to 4 engines and double the number of engines, you double the chance that there will be a catastrophic engine failure (actually its a little worse than that because engines on quads often don't get as good of maintenance attention). That increase is greater than the decrease you get in the probability that you'll have multiple engine thrust loss that turns catastrophic. No contradiction - just basic statistics.
That's not the case when you go from 1 engine to 2, but it is when you go more than 2...
Or to put it a little differently - the 767 and 747-400 were designed with roughly the same level of technology and safety, but the 767 (and 757) are statistically safer than the 747-400 (by roughly a factor of 2).