Here is an interesting answer to the question from a professional pilot:
https://www.quora.com/Are-4-engine-p...-engine-planes
The additional safety from four engines is marginal, but it is still there – that is, four engines are safer if the aircraft are of the equivalent most modern, latest certification standards.
I also found this quote from a professional pilot:
“The engines are very reliable these days, so the chance of even one of them stop functioning is low.
Four engined airplanes are more expensive to maintain and even to buy than twin engine airplanes. This is not what airlines want, they want an airplane that is very economical and reliable because it is a business and business is always about making profits.”