Inserting unique random numbers into a vector

As a part of code for a certain game I want to generate 4 unique random numbers into a vector.

This code works for some number of repeated plays and then application crashes (not responding window).

While I understand that if-condition prevents for-loop from inserting the same number into a vector, how much time does this for-loop takes until it generates unique numbers via rand() function? How srand(time(NULL)) and rand() exactly work together to create random values depending on the system time?

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <ctime>

using namespace std;

//plays bulls and cows


int main() {
srand(time(NULL));
string play="yes";
int nums=4;       // number of values in an answer (must NOT exceed 10)
vector<int> answer;


while (play=="yes" || play=="YES" || play=="Y" || play=="Yes" || play=="y") { //plays the game

answer.push_back(rand()%10+1);
  do {                              //fills vector with unique random numbers
    for (int i=1; i<nums; i++) {
      answer.push_back(rand()%10+1);
      if (answer[i]==answer[i-1]) {
        i=i-1;
        continue;
        }
      }
  } while (answer.size()!=nums);

for (int i=0; i<nums; i++) {
  cout<<answer[i];
}

  cout<<"Do you want to play again?"<<'\n';
  cin>>play;
  answer.clear();
} //game ends


if (play=="no" || play=="n" || play=="No" || play=="NO" || play=="N") { //terminates and checks for exceptions
  cout<<"Thank you for playing!"<<'\n';
  return 0;
} else {
  cerr<<"Error: wrong input. Terminating."<<'\n';
  return 0;
}

    return 0; //safety return
}