#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
int main()
{
int T;
cin >> T;
int n, m, p, q;
int ans;
while(T--) {
ans = 0;
cin >> n >> m >> p >> q;
int zu = n / m;
int yu = n % m;
if(zu == 0) {
ans = min(yu*p, q);
} else {
if(yu == 0) {
ans = min(zu * q, n * p);
} else {
ans = min(zu * q + yu * p, min((zu + 1)* q , n * p));
}
}
cout << ans << endl;
}
return 0;
}