H4 visa is issued to dependent spouses and children under the age 21 who are accompanying a legal H1 working visa holder. H4 visa can be obtained at the U.S. Consulate at the home country of the applicant. H4 visa holders are not legally authorized to work in the U.S. However, similar to requirements of H1 holders, H4 holders can study while they are in the U.S.
H4 visa holders can volunteer for a job while they are in the U.S. but not to receive any payment for it. This is an area that needs to be navigated very carefully. If the volunteer work qualifies you for a job, it is not allowed under the visa category. You can volunteer for religious and social institutions without creating a conflict. H4 visa holders can switch to other types of visas such as F1 student visa or H1B work visa provided that they meet the criteria for the respective visa category.
Some times when applied for a H4 visa in the home country, many will receive Section 221(g) status. This is not a refusal of the request. It indicates that your request need additional information to process or the Consulate need additional time to review your request.