Here are two situations:
If you want to extend your H4 status, you can do here, based on the original H1b; if you want to get an H4 visa, you have to get it from outside of the US, maybe not necessary in China, but Canada or Mexico.
If you want to extend your H4 status, you can do here, based on the original H1b; if you want to get an H4 visa, you have to get it from outside of the US, maybe not necessary in China, but Canada or Mexico.