int inputPin[]={2,3,4,5};
int sum = 0;
void setup() {
  // put your setup code here, to run once:
   for(int i = 0; i < 4; i++){
     pinMode(inputPin[i], INPUT);
   }
   Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
    int bit1 = digitalRead(inputPin[0]) * 1;
    int bit2 = digitalRead(inputPin[1]) * 2;
    int bit3 = digitalRead(inputPin[2]) * 4;
    int bit4 = digitalRead(inputPin[3]) * 8;

      sum = bit1 + bit2 + bit3 + bit4;

      Serial.print("digit:");
      Serial.println(sum);
}