I just came back from the international student office and got very useful information I personally think. Hope it can help for those with the same situation as me.
1. There is no obligation for H4 students to switch to F1 1 school year before graduation in order to get OPT. In other words, you can apply to F1 whenever you want, but just to make sure that adding the process time, you can get OPT before graduation. (This makes sense for those who go to public school).
2. Although F1 students have to take at least 12 quarter units to maintain the F1 status, the last quarter is exception.
3. 3. You can a 1-year OPT after getting a bachelor degree, get another OPT after a master degree, and one more for doctor degree. It is not that all the OPT you get add up to 1 year.
4. The counselor suggests not graduating in fall quarter, but both winter & spring are fine. (This concerns about the job opportunity and the process of applying for H1).
5. Better give a time period of 6 month for the processes of applying F1 & OPT (in total).
6. For some public school students, they can take the rest of courses in the extension of their school during the last quarter, which helps to save a lot of money.
Good Luck!