or what he means is how N9 can keep full multitasking with reasonable response (refer to cpu awake?) and minimum battery impact while n4 (nexus 4) needs to disable or close the app to keep battery life reasonable.

I think it's all about power ( and memory as well if referring to multitasking?) management that thp has mentioned.

Regarding to the android, OP may want to try an app called Greenify. It can "greenify" apps you want and put into hibernation while not used. It can somehow help to save some juice and memory (RAM)