#include <iostream>
#include <cstring>
#include <cmath>
#include <cstdio>
#include <climits>
#include <ctime>
#include <cassert>
#include <cctype>
#include <algorithm>
#include <numeric>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <vector>
#include <bitset>

#define X first
#define Y second
#define pb push_back
#define bit(x) (1 << (x))
#define bnum(x) (__builtin_popcount(x))
#define all(x) (x).begin(), (x).end()
#define mset0(x) memset((x), 0, sizeof((x)))
#define mset1(x) memset((x), -1, sizeof((x)))
#define LET(x,a) __typeof(a) x(a)
#define REP(it,v) for (LET(it,v.begin()); it != v.end(); it++)
#define sz(x) ((int)(x.size()))
#define PQ priority_queue
#define sqr(x) ((x) * (x))

using namespace std;
typedef long long LL;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pii> vpii;

template <class T> inline void chkmin(T &a, T b) {
    if (b < a) a = b;
}
template <class T> inline void chkmax(T &a, T b) {
    if (a < b) a = b;
}

const int MX = 5;

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