It would have been difficult to design it in a truly backwards compatible way, but it would have been nice for the the two protocols to be able to interoperate (ie, a packet could have an IPv4 source address and a destination IPv6 address, and vice versa). There's a draft RFC about this [1], but realistically speaking, it's probably far too late for anything to be done.

[1] https://tools.ietf.org/html/draft-omar-ipv10-01