Five way redundancy look good on paper, put if you use the same vendor and hardware purchased at the same time, you have a one point of failure, aka when one server fails there's a high probability that the others will fail simultaneously.
For highly critical systems that is an issue but it is being adressed. Take as example the space shuttle or the Airbus (3/2) primary/secondary flight computer [1] where the backup is being build by different companies with different processors ...
[1] https://ifs.host.cs.st-andrews.ac.uk/Resources/CaseStudies/A...
Same brand, same batch, operating time, running temperature, even same vibration does trick. :(
And if it's a software problem, then again the redundancy isn't helping you.
Basically you haven't succeeded in getting 5 independent tries in the problem -- the failures are highly correlated.
It's the same reasoning that led to the financial crisis -- you have a CDO or an MBS that has a bunch of different obligations so you think you've diversified away risk, but in actuality there is one big factor that causes everything to fail at once.