byte A = 0x35;
  byte B = 0xC5;
  byte C = 0x5A;
  byte D = 0x59;

  unsigned long dec_value = (D << 24) | (C << 16) | (B << 8) | A;

  char buf[50];
  ltoa(dec_value, buf, 10);  // 10 is the base value not the size - look up ltoa for avr

  String my_UID = String(dec_value);

  Serial.println(my_UID);