Thanks to all for the help.
Before questioning you here I went through many sites and forums.
In fact I already had 8 years ago the same issue on my prior +4 and managed to find the right calibration number which then helped quite a lot of +4 owners.
The Caigauge site does not help since I don't know which kind of sender I have got on my 2004 4/4 runabout. And the multiplying factor (number of teeth /of magnets / of pulses) is quite significant. That is why most people get it wrong.
In fact a 4/4a 4/4 sport, , a +4 or a roadster even when equipped with same tyres of same circumference do not have the same calibration number.
The only calibration figures I know are the following
my former 2007 +4 was 005360 (km/h)
a +8 is 004500 when speedometer in km/h
a +8 is 007400 when speedometer in mph
A shame we don't have somewhere a database of all those calibration numbers per model, mph/kph, and year...
So it's important to know the exact figure for my model. If I remember well Nigel (nputtick) has got the very same car as mine, I will try to mail him directly.
If I cannot solve my issue with recalibration then I will probably have to check the sender (where is it ?) next step would be to change the speedometer . And it looks pretty difficult to source a 4/4 MPH speedo over here.