my 2 cents
Based on what I read somewhere long time ago, H-4 is prohibited from volunteering if he, being a volunteer, occupies the position that U.S. worker can occupy and _get_paid_ for that work. The reason is that any competition for U.S. workers from foreigners has to be approved by Department of Labor in form of LCA.
I can't give you any reference, though. Take my words with grain of salt, but perhaps do a little more research on this topic...
/dzxing