Can a laptop with only an IPv6 address connect to an IPv6 Open VPN server but have an IPv4 address assigned and traverse the IPv6 connection between the IPv6 Laptop and IPv6 OpenVPN Server to access IPv4 Resources running behind the OpenVPN Server?

Yes!

The motivation to find out if this is possible is a user who holidays in India and apparently some ISP's there are IPv6 only

             +--------------------------------+
            /                                  \ IPv6 OpenVPN Server
Laptop IPv6  .6 ----- IPv4--10.99.88.0/24---- .1 ----[IPv4 Resources 10.69.0.0/24]
            \                                  /
             +---------- IPv6 Tunnel ---------+

A test ping from an IPv6 only device (I disabled IPv4 in the network settings) with Open VPN connected over IPv6 with IPv4 running inside the tunnel

ping 10.69.0.4

Pinging 10.69.0.4 with 32 bytes of data:
Reply from 10.69.0.4: bytes=32 time=13ms TTL=63
Reply from 10.69.0.4: bytes=32 time=10ms TTL=63

OpenVPN Connect client

I couldn't get the client to connect without explicitly setting the OpenVPN Server configured in OpnSense to UDP6 (UDP didn't work)

Just specifying the IPv4 Tunnel Network and the IPv4 Local Network worked (the local network is stuff behind and that is being protected by the OpnSense OpenVPN server)

The OpenVPN server address was specified as the public IP of the OpenVPN OPNSense WAN address.