overstay 4 days

I have a 14 days China Tourist Visa made in HK. I told them that my flight out of Shanghai was on september 22nd. I just discover that the last Entry date is september 5th which puts the end of the Visa september 18th or4 days overstay.
I cannot change my ticket to Switzerland, its non refundeble.
Last year I overstay one day because I counted three month 90 days, but 3 month were 91 days......
What will happen to me?
What should I do?
Thanks a lot Cornelia