It's not only that engines have gotten more efficient and powerful, its that they have also increased the reliability. The other issue is that 4 engines share almost all of the failure modes that 2 engines do. If you have 2 engines that fail that are on opposite sides of the aircraft, having 2 others isn't going to help as its likely a system failure in the aircraft or a fault in that particular model of the engine that could affect all of them. For example, if you run of fuel or have a failure in the fuel delivery system, it's not going to matter if you have 2, 4 or more engines. The mistake is thinking the probability of a 4 engine failure is significantly less than 2 engine failure for all types of failures.