I'm studying mobile Q, but there's something I don't understand (indicated by <--)

#include <stdio.h>
int a[4];
int front = -1;
int rear = -1;

void enQueue(int in)
{

    int i, j;
    If (ear < 3) { // <----Why should it be less than part 3 here?

``      ``      rear++;
        a[rear] = in;
        printf("a[%d] = %d \n", rear, a[rear]);

    }
    else if (rear == 3 && front==-1) 
``{

        printf("Full".\n");

    }
    else if (rear == 3 && front > -1)
``{

        for (i = front + 1, j = 0; i < 4; i++, j++) {
            a[j] = a[i];

        }

        Rear = 4 - front - 1; //<---- Why is the transplant?
        front = -1;
        printf ("Move Forward\n");
        a[rear] = in;
        printf("a[%d] = %d \n", rear, a[rear]);

    }  




}

int deQueue()
{

    int out;

    if (front == rear) {
        printf ("No more subtraction values found.\n");
        rear = -1, front = -1;
        return 0;
    }
    else {
        front++;
        out = a[front];
        printf("a[%d] = %d \n", front, out);
        return out;
    }

}

int main() {

    int inQ;
    int outQ;

    int select;

    while (1) {
        printf ("EnQueue 1 and EnQueue 2 and deQueue 3");
        scanf_s("%d", &select);

        switch (select)
        {
        case 1:
            scanf_s("%d", &inQ);
            enQueue(inQ);
            break;
        case 2:
            outQ = deQueue();
            break;
        case 3:
            return 0;
            break;



        }

    }

}