"Hatoon al-Fassi, Amal al-Harbi, Maysaa al-Manea, and Abeer Namankani were temporarily released," London-based rights group ALQST said on Twitter on Thursday, adding there were unconfirmed reports about the release of a fifth activist.
Another campaigner close to the families of 11 women on trial told the AFP news agency that five women had been temporarily released.
There was no immediate comment from Saudi authorities.
The activists, some of whom have accused interrogators of sexual abuse and torture during nearly a year in custody, face charges that include contact with foreign media, diplomats and human rights groups.
Three of them - activist Aziza al-Yousef, blogger Eman al-Nafjan and preacher Rokaya al-Mohareb - were granted temporary release in late March.
Riyadh has faced pressure from Western governments to release the women, most of whom were detained last summer in a wide-ranging crackdown against activists just before the historic lifting of a decades-long ban on female motorists.
Renewed crackdown
But last month, Saudi Arabia mounted a fresh crackdown that sent shock waves through the kingdom.
Authorities arrested at least nine writers and academics, including two US citizens, in an apparent crackdown on supporters of the female activists on trial, campaigners said.
It marked the first major crackdown since the brutal murder of journalist Jamal Khashoggi inside the Saudi consulate in Istanbul last October, which sparked unprecedented international scrutiny of the kingdom's human rights record.
The siblings of activist Loujain al-Hathloul - one of the prominent detainees who was not among those freed on Thursday - have said they were being pressured by people close to the Saudi state to stay silent over her treatment in detention.
Hathloul was among the women who claimed to be tortured and sexually harassed in detention.
Saudi prosecutors have roundly rejected the accusation in the ongoing trial.