#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <time.h>
using namespace std;
int n, m, p, q;
int main()
{
    //freopen("out", "w", stdout);
    //freopen("in", "r", stdin);
    int cases;
    scanf("%d", &cases);
    while(cases--) {
        scanf("%d%d%d%d", &n, &m, &p, &q);
        double val = q/m;
        int ans = 0;
        if(val < p) {
            ans = q*(n/m);
            if(n%m) {
                ans += min(q, p*(n%m));
            }
        } else {
            ans = n*p;
        }
        cout << ans << endl;
    }
    return 0;
}