Your full setup is not known so I have to make some assumptions based on a typical setup:
- Your local computer has both an IPv4 and IPv6 address in the local network.
- Your router has both a public IPv4 and IPv6 address.
- Routes on your local computer are setup so that both internet facing IPv4 and IPv6 traffic is going through the router.
- Your local computer is the start point of the VPN connection.
- The IPv4 only VPN software is not aware of the IPv6 capabilities of your computer, i.e. does not deal with them in a specific way. This is typical for OpenVPN installations.
In this setup the typically a new route (or multiple routes) for IPv4 traffic will be setup and will point to the VPN so that all IPv4 traffic will be send through the VPN. Since IPv6 traffic will not be specifically handled the existing IPv6 route will remain, i.e. IPv6 traffic will still flow outside the VPN. You can verify with sites like test-ipv6.com which IP addresses are visible from outside.
Of course, this is only true under the above assumption. It might well be that the VPN will simply disable all IPv6 on your host so that leakage using IPv6 is not possible. But, IPv6 leakage is a common problem with VPN setups.