Four digits from one 74HC595 (pins 11, 12, 13 and 7 to 4), on an Arduino Uno

// Four digits from one shift register: multiplexing.
//
//   74HC595 DS -> pin 11   ST_CP -> pin 12   SH_CP -> pin 13
//   Q0..Q7 -> 220 ohm each -> segments a..g and dp
//   digit 1..4 common anodes -> pins 7, 6, 5 and 4
//
// The segments are shared by all four digits, so the sketch shows one digit
// at a time: blank the segments, choose a digit by raising its anode, send
// its pattern, wait a few milliseconds, and move to the next. Round and round
// fast enough, all four look lit at once.

const int LATCH = 12;  // ST_CP
const int CLOCK = 13;  // SH_CP
const int DATA = 11;   // DS

void write595(byte value, int order) {
  digitalWrite(LATCH, LOW);
  shiftOut(DATA, CLOCK, order, value);
  digitalWrite(LATCH, HIGH);
}

// The digits 0 to F for a common-anode display: a 0 bit lights a segment.
// Bit 0 is segment a, bit 6 is g and bit 7 is the decimal point.
const byte DIGITS[16] = {0xC0, 0xF9, 0xA4, 0xB0, 0x99, 0x92, 0x82, 0xF8,
                         0x80, 0x90, 0x88, 0x83, 0xC6, 0xA1, 0x86, 0x8E};

const int DIGIT_PINS[4] = {7, 6, 5, 4};

void choose(int digit) {
  for (int i = 0; i < 4; i++) digitalWrite(DIGIT_PINS[i], i == digit ? HIGH : LOW);
}

void setup() {
  pinMode(LATCH, OUTPUT);
  pinMode(CLOCK, OUTPUT);
  pinMode(DATA, OUTPUT);
  for (int i = 0; i < 4; i++) pinMode(DIGIT_PINS[i], OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: four digits counting up in hex");
}

void loop() {
  unsigned int value = millis() / 100;
  for (int i = 0; i < 4; i++) {
    int nibble = (value >> (4 * (3 - i))) & 0x0F;
    write595(0xFF, LSBFIRST);
    choose(i);
    write595(DIGITS[nibble], LSBFIRST);
    delay(4);
  }
}