I want to make a serial communication --- 18f4550 and max6675 ---

but max6675 demand 4,3 mhz max to have the correct communication

how can i run my program on pic18f4550 at 4 Mhz

(the pic is also connected to pc via usb) ,the probleme is how change this default configuration to have programme running speed at 4 mhz and keep usb communication at 48Mhz.

__CONFIG _CONFIG1L, _PLLDIV_5_1L & _CPUDIV_OSC1_PLL2_1L & _USBDIV_2_1L
__CONFIG _CONFIG1H, _FOSC_HSPLL_HS_1H
__CONFIG _CONFIG2H, _WDT_ON_2H & _WDTPS_512_2H
__CONFIG _CONFIG3H, _PBADEN_OFF_3H
__CONFIG _CONFIG4L, _LVP_OFF_4L & _ICPRT_OFF_4L & _XINST_OFF_4L