This is what I was told when I was in school
Lets say engine reliability = 0.99
2 engine: 0.99^2 = 0.9801
4 engine: 0.99^4 = 0.9605
i.e. 4 engine is less reliable in engine terms than a 2 engine. Thats how it has been since forever. What changed over time is engine reliability, thus what failure mode is more important (engine failure itself or outcome of engine failure)
An engine failure can be anything. Fuel pump or turbine disk failure (uncontained)
A fuel pump failure is unlikely to create further problems. A turbine disk failure may create more problems. But both are engine failures. See recent A380 or old times DC-10 ( the one that burst the hydraulic lines).
Back in the days that engines were less reliable, more engines were a short of 'redundancy' because they were unreliable (compared to nowadays).
Now that engine reliability is higher, probability of losing engine is low, thus you don't need many engines cause even if you lose one (extremely unlikely event), its extremely unlikely that you loose one again (due to engine reliability).
Problem arises by the likelihood of damage to other systems due to engine failure (for example turbine disk failure). In that respect a 4 engine plane is more likely to get damaged due to one of its engines failing, because it has more engines. Furthermore engine failure of any short is distracting to the crew, ATC etc etc, i.e. other things can go wrong and line up the holes.
So nowdays with extremely reliable engines, there is less benefit from engine redundancy (3-4 instead of 2) because adding more engines reduces the reliability of the overall system (plane). That doesn't mean that its not safe to have more engines, it means that there is a trade off between 'how many engines' and 'what do I have to gain'. Back in the days you gained redundancy, nowadays you gain other things. The only sure thing is that engines are more reliable and that makes aviation safer and planes better.
If the fuel is contaminated, or if the same person installed all fuel pumps wrong in all engines is a different story and there are other safeguards against that. If the same person installs and checks all the fuel pumps on a 2 or a 4 or a 256 engine plane, then all engine reliability figures fly off the window because chance of engine failure is not based on independent probabilities anymore.
btw, I'm not saying that the number of engines during design depends only on the above. The above are the reasons that ETOPS became possible.