Hmm, something has changed:

Something has changed since I originally had this problem. Using waitip 4 in /etc/dhcpcd.conf appears to now work for what I need. If you are having problems with the Network not being up in time for services that require an interface with an ipv4 address, you should first try the above parameter.

I'll leave this here for a historical reference. The dhcpcd.entry-hook can be useful, if you need to update some information with the current IPv4 address.

ifup-wait-dhcpcd

This work around applies to Raspbian. It attempts to force a delay on network-online.target being available, until we have an IP Address. Includes a 30 second timeout to avoid a possible hang on Network failure. See Wiki