I'd say you should talk to lawyer before any volunteer or intern work under H4.
If it's real volunteer work, it should not have any problem, but if it comes to "productive work" which all the profittable company is doing, it is in gray zone. It might be considered taking away the job from the US citizen, and if so, "working" under H4 even w/o pay beats the purpose of all the work authorization like H1 or L1.
It's gray, but I think it's more on black side.