Output of a simple number guessing game in c using while and do-while loop

I am a beginner in C programming and I want some help with a right digit counter in a number guess game. In this game, a random secret number of 4 digits is generated and user has to guess it by entering different numbers. In this code, each digit of the user input is scanned and checked. If a digit of the secret number is found in input, the counter k adds 1 and so it should give number of guessed digits. Order is not important (at this stage). Problem: the game gives a smaller number of digits guessed.

#include<stdio.h>
#include<stdlib.h>
#include<time.h>

int main(){
setvbuf(stdout,NULL,_IONBF,0);

int n=10000;
srand(time(NULL));
int r=rand()%n;//makes a random (secret) number

int d1,d2,d3,d4;//declares digits of the random number

d1=r/1000;//these 4 lines calculate separate digits of the random number
d2=(r-d1*1000)/100;
d3=(r-d1*1000-d2*100)/10;
d4=r-d1*1000-d2*100-d3*10;

char c;//declares char c which will be a digit of a number given by user (guess)

if(d1!=d2&&d1!=d3&&d1!=d4&&d2!=d3&&d2!=d4&&d3!=d4){//prevents using a random number which has some duplicate digits

 printf("Enter a number:");
 scanf("%c",&c);//scan first digit(character) of user's guess

 while(c!='\n'){//scan all digits(characters) of user's guess until new line
 int k=0;//initialize a counter of right guesses

 while(c!='\n'){//scan all digits(characters) of user's guess until new line
  c=getchar();//scan each character of a user's number
  int digit=c-48;//convert the character into digit by using its ASCII value

   if(digit==d1){//if user digit coincides with the first digit of random number add 1 to counter
    ++k;
   }
   else if(digit==d2){//if no, check if it coincides with second digit
    ++k;
    }
   else if(digit==d3){
    ++k;
   }
   else if(digit==d4){
    ++k;
   }

  }

 printf("number of guessed digits is %i\n",k);
 printf("secret number =%i\n\n",r);

 printf("Enter a new number:");//asks the user to try another number
 scanf("%c",&c);//scan new digit
 }
}

return 0;

}

Example of output:

Enter a number:2015 number of guessed digits is 2 secret number =4901

Enter a new number:4902 number of guessed digits is 2 secret number =4901

Enter a new number:4901 number of guessed digits is 3 secret number =4901

Enter a new number:

Thanks for help in advance!