@EasyB: Thanks for the advice! The voltage meter from the power supply is totaly out of scope...
I soldered directly thick wires to the 20AWG wires and took short (25cm) 4mm 2.5mm2 (AWG14) cables, stronger clamps and measured the voltage with a DMM:
Now I get @3.4v 4A, @3.7v 6A.
Does this new luxeon V need a "burn in time" like the old Luminus SST50/SST90 to drop the needed voltage?
I can’t quite picture exactly your measurement set-up, but I might suspect you still have voltage drops you are not accounting for, like at the clamps. It doesn’t take a lot of resistance to drop 0.3V at 6A.