Infinity Ward has changed Possum, the DMZ trait that can revive a downed player in Call of Duty: Modern Warfare 4. Per the official patch notes, you can't activate Possum "while the team that killed you is nearby or until 4 minutes have passed", and the studio says the range and duration are "tunable going forward".
"The intention for Possum is to give a high level operator a second chance. With the current iteration, Possum was being used as a combat mechanic instead of a survival mechanic," the notes say. The same update fixes a bug where Possum could revive players who had already abandoned their Operator, and a black screen when a whole squad went down while a teammate had Possum equipped.
It also fixes the 3D Printer bug that could send players into DMZ without their Free Loadout weapons or equipment. Eurogamer also reported the change.
Sources: Call of Duty · Eurogamer