#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long ll;
const int N = 100+5;

int main() {
    int t;
    int n, m, q, p;
    scanf("%d", &t);
    while(t--) {
        scanf("%d%d%d%d", &n, &m, &p, &q);
        if(p*m < q) printf("%d\n", n*p);
        else {
            int ans = min((n+m-1)/m*q, n/m*q + (n%m)*p);
            printf("%d\n", ans);
        }
    }
    return 0;
}