Currently H4 visa holders are not allowed to work. Even if they're applying for permanent residency with the primary applicant, they can't obtain employment authorization until they're eligible to file their I-485.
Due to retrogression of visa number availability, this situation can last for many years. H4 holders face loneliness, depression, loss of self-identity and strained marital relations (arising from complete dependence on their spouse).
If H4 holders who already have an approved I-140 could seek employment, it would help many families survive intact and relieve considerable suffering.