#include <cstdio>
#include <algorithm>

using namespace std;

int n,m,p,q;

int main() {
    int t;
    scanf("%d",&t);
    while (t--) {
        scanf("%d%d%d%d",&n,&m,&p,&q);
        if (p*m<q) q=p*m;
        printf("%d\n",min(n/m*q+n%m*p,n/m*q+q));
    }
    return 0;
}