IPv4 is defined by RFC 760 and RFC 791, although I don't know if RFC 760 actually implements something that looks like modern IPv4 (one of the challenges with old RFCs is that they were actually requests for comment, and I don't have a good sense of when this practice actually stopped).
I was going by Wikipedia's dates, which give the actual use of IPv4 on ARPANET as January 1983, which I believe postdates the actual use of SMTP on ARPANET.