I know this might sound basic to a lot of you but in school I've started taking lasers and laser mechanics in my physics course and so we reached the whole four-level system and three-level system lasers and I have a question.. Why do four-level systems require less energy than the third-level systems? As far as I know, electrons can go to higher energy levels (excited) if they have more power. So how would I send an electron from E1 to E4 with less pumping power than from E1 to E3 ? Please try to keep the answer as simple as possible because English isn't my main language so I might have some difficulties with large words. Thanks in advance!