I finally succeeded in programming correctly the throttles via FSUIPC.
It is mandatory to set ExclThrottleSet=Yes
The correc program for the virtual pilots using 4-engines Throttle quadrant is the following :
[JoystickCalibration.iFly 747-400 LH - Virtual Cockpit/Cabin]
ExclThrottleSet=Yes
SepRevsJetsOnly=No
FlapsSetControl=0
ReverserControl=66292
Reverser1Control=66422
Reverser2Control=66425
Reverser3Control=66428
Reverser4Control=66431
MaxThrottleForReverser=0
AileronTrimControl=0
RudderTrimControl=0
CowlFlaps1Control=0
CowlFlaps2Control=0
CowlFlaps3Control=0
CowlFlaps4Control=0
Elevator=-16383,0,0,16383
Rudder=-16380,0,5,16380
Throttle=-15337,16192
Throttle1=-16129,-14194,-14194,16129
Throttle2=-16193,-14258,-14258,16192
Throttle3=-16193,-14258,-14258,16192
Throttle4=-16256,-14321,-14321,16256
Flaps=-14913,16192
Spoilers=-16129,16129
SlopeSpoilers=1