#4, because its the honest one. why do you need to make up and excuse to quit your job.
if you're quitting just speak whatever it is and leave the company
if i am a student working on a part time job, i will select (1)...if i am working, i will just to honest with my employers and told them that i got a better job and here is my 2 weeks notice.
If I were still young, I will frankly say the salary is too low, and quit. Go to find another better pay job, as I can still go back to enrich myself by more study. But remmeber, be keeping a good image with your boss and colleagues.