Regarding Initialization in Embedded C for MSP430F6779

#define P4DIR_INIT  (BIT0 | BIT2)

This means, when I say P4DIR_INIT, I really mean (BIT0 | BIT2).

Hence subsequently, when I say:

P4DIR |= P4DIR_INIT;

I really mean:

P4DIR |= (BIT0 | BIT2);

By the way, P4DIR,  BIT0, and BIT2 are not C keywords either. They are all defined somewhere else. As a matter of fact. one could have done the following instead:

#define Direction_configuration_of_Port4 P4DIR |= 0x03;

Then use it for Direction configuration of Port 4 with:

Direction_configuration_of_Port4;