How to extend my Sketch for Multiple Inputs and Outputs
I'll show you the long but simple way. Just follow this logic in your setup and loop and you'll be ok. If you still don't understand and want more, tell me and I'll add more code for you.

Do something like this:

const int buttonPin1 = 2;    // the pin the button is connected to
const int buttonPin2 = 3;
const int buttonPin3 = 4;
const int buttonPin4 = 5;

const int ledPin = 6;        // the pin the led is connected to
const int ledPin = 7;
const int ledPin = 8;
const int ledPin = 9;

int ledState1 = HIGH;         // the current state of the output pin
int buttonState1;             // the current reading from the input pin
int lastButtonState1 = LOW;   // the previous reading from the input pin
long lastDebounceTime1 = 0;  // the last time the output pin was toggled
long debounceDelay1 = 50;  

int ledState2 = HIGH;         // the current state of the output pin
int buttonState2;             // the current reading from the input pin
int lastButtonState2 = LOW;   // the previous reading from the input pin
long lastDebounceTime2 = 0;  // the last time the output pin was toggled
long debounceDelay2 = 50;  


int ledState3 = HIGH;         // the current state of the output pin
int buttonState3;             // the current reading from the input pin
int lastButtonState3 = LOW;   // the previous reading from the input pin
long lastDebounceTime3 = 0;  // the last time the output pin was toggled
long debounceDelay3 = 50;  

int ledState4 = HIGH;         // the current state of the output pin
int buttonState4;             // the current reading from the input pin
int lastButtonState4 = LOW;   // the previous reading from the input pin
long lastDebounceTime4 = 0;  // the last time the output pin was toggled
long debounceDelay4 = 50;