In MAP-E and MAP-T specifications, the only fully described behaviors involves datagram reassembly at domain entry before forwarding.
[7] The first "4rd" specification, unlike the current one of RFC 7600, used IPv4 encapsulation in IPv6 packets, the only known tunneling approach at that time to ensure complete IPv4 preservation across IPv6-only domains.
Compared to encapsulation, it had the advantage of making IPv6 packet inspections applicable to translated UDP and TCP IPv4 packets, but, due to limitations of SIIT, lacked full compatibility with IPv4 fragmentation (and consequently, as mentioned above, compatibility with path MTU Discovery recommended in RFC 6349).
[12] Finally, in December 2014 the Softwire working group[15] changed its previous decision, and decided to put MAP-T on Standards Track in parallel with MAP-E, provided a note in the MAP-T RFC would signal its incompatibility with the path MTU Discovery of RFC 4821.
The French ISP Free is deemed to have deployed 4rd for its experiment of FTTH in "lesser-dense areas", starting from December 2015.