const int MOTOR[] = {3, 4, 19, 38};
const int PWM12 = 40;
const int PWM34 = 39;

void setup() {
    for (int i = 0; i < 4; i++)
        pinMode(MOTOR[i], OUTPUT);
    
    // enable motor output with pulse width modulation turned on all the time
    pinMode(PWM34, OUTPUT);
    pinMode(PWM12, OUTPUT);
    digitalWrite(PWM34, HIGH);
    digitalWrite(PWM12, HIGH);
}

void loop() {
    const char halfsteps[ ] = {0x9, 0x1, 0x5, 0x4, 0x6, 0x2, 0xa, 0x8};
    static int i;

    digitalGroupWrite(MOTOR, 4, halfsteps[i++ & 7]);
    delay(50);
}

// write to a group of output pins
void digitalGroupWrite(const int* group, int length, int data)
{
    for (int i = 0; i < length; i++)
        digitalWrite(group[i], data & (1 << i) ? HIGH : LOW);
}