Hi Pietro,
Why do you change the divider when you use the FBI?
If you go to Reference Manual, page 593
http://cache.freescale.com/files/microcontrollers/doc/ref_manual/K64P144M120SF5RM.pdf

so, the value for fcrdivVal = 6, output frequency cannot be 32KHz.
Because when the value is 5, 4MHz and Divide Factor is 32, is 125KHz. It is right.
BUT: when is the value 6, 4MHz and Divide Factor is 64, it is 62,5KHz, not 32KHz.
(4MHz : 64 = 62,5KHz)
As it is written in the Reference Manual : you have not change the divider when the Fast IRC is enabled.
Please, look to SIM Module, Register SIM_CLKDIV1->OUTDIV4 what is your frequency divider for flash memory.
Best Regards,
Iva