#include<stdio.h>

#include<math.h>

#include<string.h>

#include<stdlib.h>

#define N 110

using namespace std;

int main()

{

    int t, sum, n, m, p, q, a, b;

    scanf("%d", &t);

    while(t--)

    {

        sum = ;

        scanf("%d%d%d%d", &n, &m, &p, &q);

        a = n / m;

        b = n % m;

        if((n - b) * p > a * q)

            sum += a * q;

        else

            sum += (n - b) * p;

        if(b * p > q)

            sum += q;

        else

            sum += b * p;

        printf("%d\n", sum);

    }

    return ;

}