PIC Alternate Pin Function for PWM
If it could do PWM on four channels they would brag about it in the datasheet, it's a selling point. As it stands, it has three channels of PWM. When you try to move a PWM channel from one pin to another, the other pin would not be a PWM anymore.

Section 11.1 in the datasheet:


  the following functions can be moved between different pins.


If you want a fourth PWM channel you have to modulate it by software using one of the timers. Sometimes the USART can be put to clever use here, loading the serial shift register with a bit pattern such as 11100000 and selecting a high enough baud rate.