Been using the Pi1 for a couple years now with OpenVPN and Pihole, never had problems (though the only one using the VPN is me, at any time), so I'd say the Pi4

Been using the Pi1 for a couple years now with OpenVPN and Pihole, never had problems (though the only one using the VPN is me, at any time), so I'd say the Pi4 should be more than capable :-)