I believe the FAA must ensure that all airplanes on commercial operations whether twins, tris or quads are equally safe.
It appears we are fixated on the number of engines and the flexibility they offer when an engine fails.
Based on what I have read, it is my understanding that ETOPS covers more than engine issues.
I understand the FAA could not ignore the issue of tris/quads decompression enroute long-range operations and the resulting need for additional fuel reserves. (Remember Qantas 747 decompression and Manila diversion, luckily the incident happened close to an airport, imagine the same scenario on Sydney-Santiago!!)
I believe, similar to ETOPS for twins, ETOPS for tris /quads will require fuel planning for such eventuality (decompression). Airlines can address this by carrying additional fuel. If they do not want to carry additional fuel and forgo the payload then they will have to look for a suitable alternates enroute. Based on the limited knowledge I have on ETOPS, it appears to me that ETOPS greatly enhances the overall safety of all operations including those of tris & quads.