IPv6 clients (or in theory any kind of IPv4 successor) can reach IPv4 servers via some kind of translation layer (for example NAT64) - so IPv6 is backwards-compatible with IPv4 in that direction. The inverse direction (IPv4 client to IPv6 server) is however not possible, since IPv4 is not forward-compatible with any possible successor, because it is not possible to encode more information into 32-bit than 32-bit.