>4 smaller but equally as efficient engines
Larger jet engines are more fuel efficient than smaller ones, because larger diameters allow for more bypass air and therefore more fuel efficiency [1]. It is a function of size and a lot of the engineering goes into materials and designs to be able to increase size and maintain strength. So you simply can't make 4 engines that are as efficient as 2 large ones, and that is compounded by the significant additional weight (and drag) of the duplicated engine parts and mounting structures.
>if some of those can be avoided by having 4 engines for commercial aircraft, its a worthwhile idea to explore
Everything comes with tradeoffs. Adding more engines mean more complexity, more maintenance, more chance of single engine failures, etc. You don't want to introduce more failure modes than what you are trying to fix. The move to two engines for large aircraft and the evolution of ETOPS (Extended Twin-engine Operational Performance Standards) involved a lot of people considering a lot of scenarios. I can guarantee the "why not 4 engines" question has been studied extensively.
[1] https://simpleflying.com/why-do-jet-engines-keep-getting-lar...