Get your Breadboard out.

Put buttons on Breadboard on the center.

On each Button put a resistor going to ground.

Attach Ground and Power(5V).

On the side of the Buttons that you put the resistor put a cable in.

Attach the cable to pins 2,3,4, and 5.

Put power to the other side of the buttons(where you didn't put ground)

Power on Raspberry Pi and attach the Arduino to it!

Upload this sketch to Arduino:

void setup() {

  Serial.begin(9600);

  pinMode(2,INPUT);

  pinMode(3,INPUT);

  pinMode(4,INPUT);

  pinMode(5,INPUT);

}

void loop() {

  Serial.println(digitalRead(2));

  Serial.println(digitalRead(3)+2);

  Serial.println(digitalRead(4)+4);

  Serial.println(digitalRead(5)+6);

  delay(300);

}