ChristiaanJ
I explained data streaming ( continuous down linking via Iridium satellites of aircraft parameters ) in some post in the original AF447 thread .
Details can be found at
AeroMechanical Services Ltd. - afirs Up Time
The system already is in place but mainly for VIP jets as it is expensive indeed.
Also , as you very correctly pointed out, the last seconds , possibly even the last minute, would probably be missing in our case.
Another idea would be to have a small amount of radioactive material being released inside the recorders after a certain depth ( say 1000m or so ) enabling some military sensors to detect the anomaly and the location. Not sure of the accuracy of those sensors though. Possibly something for the future.
As to the duration of the "PINGS " I was told they weaken with time , but still can be read after a very, very long time. Much longer than 28 days.
The recorders of the KAL o7 B747 shot down over Sakhalin island was retrieved by the Russian Navy more than a year after its demise.
But they were not laying down at 4000m .