Arduino sketch for intercepting digital inputs of four buttons and broadcasting a corresponding serial value via Bluetooth.

bluetooth.ino

const char BUTTON_CHARS[] = {(char)48, (char)49, (char)50, (char)51};
const int BUTTON_COUNT = 4;
const int BUTTON_PINS[] = {4, 5, 6, 7};
int button_states[] = {0, 0, 0, 0};
int button_last_states[] = {0, 0, 0, 0};

void setup() { 
  pinMode(BUTTON_PINS[0], INPUT);
  pinMode(BUTTON_PINS[1], INPUT);
  pinMode(BUTTON_PINS[2], INPUT);
  pinMode(BUTTON_PINS[3], INPUT);
  Serial.begin(115200);
}

void loop(){
  
  for(int i=0; i<BUTTON_COUNT; i++){
    button_states[i] = digitalRead(BUTTON_PINS[i]);
    
    if(!isOtherButtonPressed(i) && button_last_states[i] != button_states[i]){
      if(button_states[i] == HIGH){
        Serial.write(BUTTON_CHARS[i]);
      }
    }
  }
  updateLast();
}

void updateLast(){
  for(int i=0; i<BUTTON_COUNT; i++){
    button_last_states[i] = button_states[i];
  }
}

boolean isOtherButtonPressed(int index){
  for(int i=0; i<BUTTON_COUNT; i++){
    if(i!= index && button_states[i] == HIGH){
      return true;
    }
  }
  return false;
}