Can I use the arduino for an application with a frequency of 4 MHz?
I need to create a clk with this frequency and send and receive data on the rising and falling edges. and it is not a normal SPI interface I have my own conditions so I need to do it manually.
If it is not suitable, is it technically possible?
The maximum PWM you could generate with an arduino Mega 2560 is 62500 Hz. I don't think you can go beyond that. Method
You can use the internal SPI so with a 16MHz oscillator you could have upto some where from 16MHz to (16/128)MHz. Method
You can PWM output at 4MHz with the Timer1 (ATMega328 and ATmega32U4), here is an example for UNO/NANO (ATMega328):
NOTE: Any Arduino code that uses TIMER1 won´t work after this (or will work erratically). Arduino uses TIMER1 for Servo library.