/*
 *     Date:
 *        2015/07/25
 *     Author:
 *        Indestinee
 */
#include <cmath>
#include <cstdio>
#include <string>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
int cas, n, m, p, q;
int main(){
//    freopen( "" , "r" , stdin );
//    freopen( "" , "w" , stdout );
    cin >> cas;
    while( cas -- ){
        scanf( "%d %d %d %d" , &n , &m , &p , &q );
        if( p * m <= q ){
            printf( "%d\n" , n * p );
            continue;
        }
        int ans = min( n / m * q + ( n % m ) * p , ( n + m - 1 ) / m * q );
        printf( "%d\n" , ans );
    }
    return 0;
}