int  D1 = A3;
int D2 = A2;
int D3 = A1;
int D4 = A0;
unsigned char binary_pattern[]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};
unsigned int a,b,c,d;
unsigned int counter = 0;

void setup() 
{
  pinMode(D1, OUTPUT);  
  pinMode(D2, OUTPUT);  
  pinMode(D3,  OUTPUT);  
  pinMode(D4, OUTPUT);  
  DDRD = 0xff;
  
}
 
void loop()
 {
  digitalWrite(D1, HIGH);
  digitalWrite(D2,  HIGH);
  digitalWrite(D3, HIGH);
  digitalWrite(D4, HIGH);
  while(1)
    {
        
           a = counter / 1000;   // holds 1000's digit
           b = ((counter/100)%10); // holds 100's digit
           c = ((counter/10)%10);  // holds 10th digit
           d = (counter%10);  // holds unit digit value
        
PORTD=binary_pattern[a]; // send 1000's place data to fourth digit
D1=0;   //  turn on forth display unit
delay(10000);
D1=1;   //  turn off forth display unit
PORTD=binary_pattern[b];  // send 100's place data to 3rd digit
D2=0;    //  turn on 3rd display unit
delay(1000);
D2=1;  //  turn off 3rd display unit
PORTD=binary_pattern[c];  // send 10th place data to 2nd digit
D3 = 0;  //  turn on 2nd display unit
delay(1000);
D3 = 1;   //  turn off 2nd display unit
PORTD=binary_pattern[d];  // send unit place data to 1st digit
D4 = 0;  //  turn on 1st display unit
delay(1000);
D4 = 1;  //  turn off 1st display unit

if(counter>=1000) //wait till flag reaches 100
{
    counter=0; //only if flag is hundred "i" will be incremented 
}
counter++; //increment flag for each flas
    }
    return ; 
  
}