In this scenario, R sends packets to S over IPv4 only. So S must have an IPv4 address? If S has an address that is larger than 32 bits, what goes in the "destination IP" field in the IPv4 packet that R is capable of sending? If S has an address that is guaranteed to be less than 32 bits then we're just in the IPv4 world.

How are R and S negotiating when R cannot even name S on the network? Its stack only allows for 32-bit addresses and S can't have a 32-bit address.