#include <cstdio>
#include <algorithm>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>

using namespace std;

void dit(){
    int n,m,p,q;
    scanf("%d%d%d%d",&n,&m,&p,&q);
    q=min(q,p*m);
    printf("%d\n",(n/m)*q+min((n%m)*p,q));
}
int main() {
    int T;scanf("%d",&T);while(T--)dit();
}