If the I-130 takes 4-5 years, so the whole process will take 4+12=16 years for F4 category?
-No.
or the total is about 12 years including the I-130's 4-5 years?
-Yes.
what date is the priority date? the date I submit I-130 or I-130 get approved (my understanding is this one)?
-It is the I-130 receipt date.
I understand only when Priority date is current, then the applicant can submit I-485.
So how to calculate CSPA age?
-It is: the child's CSPA age = the child's actual age when the priority date is current (排期到時)- I-130 pending dates.
Note: the I-130 pending dates = I-130 approval date - I-130 receipt date
If the child's CSPA age is < 21, then the child can file I-485 in US or file DS 260 to apply for the immigrant visa in China. Once the child files I-485 or DS 260, his/her CSPA age will be frozen. Thus he/she will have no age-out problem at all.
From above equation, you can see the later the I-131 is approved, the less age-out risk for the child (i.e. the better to the child).
is that the time when I-130 get approved? or the time to submit I-485? is the freezing age protection covering this F4 catagery (sister's son) or just only for citizen/GC's child?
Thank you again for your help!