If the mapping between the IPv4 address and MAC address is found in machine A's ARP cache, then you can guess some of machine B's IPv6 addresses (usually one or two) as follows:
- The Link local address, which is fe80::Modified-EUI-64. The modified EUI-64 is derived from the MAC address.
- One autoconfigured IPv6 address per subnet on the local link. Like the link local address, the bottom 64 bits of this address are the modified EUI-64.
However, note that the link local address may not be that useful (it can only be used to communicate on-link with machine A) and as for autoconfigured addresses, that function is not always enabled.
If you don't know machine A's MAC address then you can't predict anything about iuts IPv6 address(es) from its IPv4 address(es).