Thank you for the reply. The priority is highest in PortB. I am happy, that there is a solution using four external interrupts of the Mega2560 to count encoder :
#define leftEncoder1Pin 2 // Used for attachInterrupt
#define leftEncoder2Pin 19 // Used for attachInterrupt
#define rightEncoder1Pin 3 // Used for attachInterrupt
#define rightEncoder2Pin 18 // Used for attachInterrupt
attachInterrupt(digitalPinToInterrupt(leftEncoder1Pin), leftEncoder, CHANGE);
attachInterrupt(digitalPinToInterrupt(rightEncoder1Pin), rightEncoder, CHANGE);
attachInterrupt(digitalPinToInterrupt(leftEncoder2Pin), leftEncoder, CHANGE);
attachInterrupt(digitalPinToInterrupt(rightEncoder2Pin), rightEncoder, CHANGE);
This gives the same performance as Revision 1.2 with separate encoder functions. Not sure if it really better than Rev 1.2.
The defines for PIN_CHANGE_INTERRUPT_VECTOR can be deleted, but all the #if defined statements must be deleted, too