How does the ip4v1 device know how to address the ip4v2 device in order to send a packet back? > As for the routing table size issue, it's not really an issue

How does the ip4v1 device know how to address the ip4v2 device in order to send a packet back? > As for the routing table size issue, it's not really an issue at all. Assuming extra octets, a /24 block holds 16M addresses instead of 256 (or 254 realistically). You don't need a larger routing table. If you want to subdivide those 16M addresses in any useful way, you're going to need a larger routing table.