The header of an IPv4 packet has the source and destination addresses, both as 32-bit values. These fields are adjacent, and there's other stuff next to them. If you appended more bytes to the source address, routers would think that those new bytes are the destination address. This would not be backward compatible.
Interestingly, what you're describing really is similar to how many languages represent an IPv4 address internally. Go embeds IPv4 addresses inside of IPv6 structs as ::ffff:{IPv4 address}: https://cs.opensource.google/go/go/+/go1.26.2:src/net/ip.go;...
Interestingly, what you're describing really is similar to how many languages represent an IPv4 address internally. Go embeds IPv4 addresses inside of IPv6 structs as ::ffff:{IPv4 address}: https://cs.opensource.google/go/go/+/go1.26.2:src/net/ip.go;...