Four engines are at least one too many.
Three is perhaps the best compromise, but is hard to implement without serious weight penalties. Perhaps when blended wing body designs appear, a return to three will ocur.
Two is the best configuration moving forward. There are a multitude of advantages to twins that one should consider. I offer the following as food for thought. I received this secondhand and the author is unknown to me.
>>After reading the recent thread, I thought I'd offer my opinion as to why 2
engines are safer than 4 engines on any aircraft.
The main issue is performance. Each aircraft is required to meet certain
climb performance requirements on takeoff in the event of an engine
failure. The climb performance requirements are virtually the same,
regardless of whether the aircraft has 2, 3, or 4 engines. A 2 engine
aircraft will therefore have about the same performance on 1 engine as a 4
engine aircraft has on 3 engines (each has one engine failure).
Most accidents do not involve engine failures. In situations where all the
engines are operating, 2 engine aircraft will have much better climb
performance than 4 engine aircraft. This enables 2 engine aircraft to have
a much better chance at surviving encounters with windshear, or perform
GPWS escape maneuvers (to avoid hitting the ground). They will also have
better performance during a go-around.
There are also problems with 4 engine aircraft. Many people point to
instances of dual engine failures on 4 engine aircraft as an argument
against 2 engine aircraft. In reality, I would argue the opposite point.
The dual engine failures on 4 engine aircraft that I can think of are: El
Al in AMS, the United 747 in HNL, and Evergreen in Alaska. In each of
these cases the 2 engine failures were related. In both the El Al and
Evergreen cases, the explosive failure of the #3 engine (inboard right
side) caused the failure of the #4 engine. In the United case, debris from
the cargo door, etc. caused both the #3 and #4 engines to fail.
If these same events occurred on a 2 engine aircraft, only 1 of the engines
would fail. While it might appear that both aircraft have lost half of
their engines, the 4 engine aircraft is in a much more serious situation.
The 2 engine aircraft still has 100% of it's required engine out
performance; the 4 engine aircraft only has 66%. The 4 engine aircraft
doesn't even meet takeoff climb performance requirements. If the
initiating event occurs very soon after takeoff, it is quite possible the
aircraft will not be able to clear surrounding terrain.
The argument is also made that flying with 2 engines over the water is not
safe. In the modern era, there has never been an aircraft accident due to
a loss of thrust from an engine during cruise (I'm specifically excluding
UA 232, which resulted from a hydraulic problem, not a loss of thrust).
There has also never been an instance of dual, unrelated engine failures.
Under ETOPS rules, there are strict limits on the operation of the aircraft
which seriously lessen the risk of related engine failures. In fact, a 3
engine Eastern L1011 almost ditched in the water off of Miami due to
related engine problems with all 3 engines (which would not have happened
if the aircraft was operated under ETOPS rules).
In my opinion, overwater flight under ETOPS is safer. I'm not worried
about flying on 1 engine after a failure; the airplane does that just fine.
I am worried about not having an alternate to which to divert. Under
ETOPS, there are strict rules on diversion airports, including the weather
at the alternate, and the time required to get there. There are no such
limitations for non-ETOPS flights. If an airplane is on fire out over the
water, it won't matter how many engines there are. The availability of an
alternate airport might make a difference, however.
2 engine aircraft are much better equipped to survive windshear and CFIT
encounters, two of the leading accident causal factors in recent years.
Unrelated independent engine failures is not a leading accident causal
factor. In addition, 2 engine aircraft under ETOPS are better protected
against related engine failures than 4 engine aircraft. That is why I feel
safer boarding a 2 engine aircraft.<<