回複：請教xiaobaitu：H4簽證持有人可否在美擔任國內客戶的代理？

When you sign something for somebody, you will also be taken responsible for the papers. I do not think H4 can be involved in this kind of activity.
You may be able to let them pay you in China but H visa holders are supposed to report their global incomes.
So you are looking at two things. Legal representation and tax issues.
To be quite honest, I think you will be violating immigration and tax laws.
If there is a tiny thing goes wrong, you will be in big big trouble.