Upgrading MacBook Pro mid-2009 RAM
My bad! It was a matter of the frequency.
The 4 gb cards are 1600 MHz and the max admitted by mid-2009 is 1066 MHz.
When installing 2 gb (1066 MHz) + 4 gb (1600 MHz), the macbook automatically slows down the speed of the highest to the lowest one. But if both 4 gb are installed, the Macbook gives the error. That's it.
Thank you for the advises.