Sendmail (up to 13.2 in base system) does not have an option to stop using IPv6 when sending email, but you can use pf to block IPv6 smtp traffic. Try something like this:
Then test it by telnet. If you see permission denied, then sendmail will use IPv4 for sure.
/etc/pf:
ext_if="em0"
block out quick on $ext_if inet6 proto tcp from self to any port 25
Then test it by telnet. If you see permission denied, then sendmail will use IPv4 for sure.
% telnet -6 aspmx.l.google.com 25
Trying ..........
telnet: connect to address ..........: Permission denied
telnet: Unable to connect to remote host