/*
Bismillahir Rahmanir Rahim
(In the name of Allah)

@Author Muhammad Nadim
*/

/* Start of bits/stc++.h template */

// C++ includes used for precompiling -*- C++ -*-

// Copyright (C) 2003-2015 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file stdc++.h
 *  This is an implementation file for a precompiled header.
 */

// 17.4.1.2 Headers

// C
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdalign>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif

/* End of bits/stdc++.h template */

using namespace std;

#define ll long long
#define ld long double
#define nl '\n'
#define pi acos(-1.0)
#define sp(n) fixed << setprecision(n)
#define MNTDH_18                      \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);
#define yes cout << "YES" << "\n"
#define no cout << "NO" << "\n"
#define all(v) v.begin(), v.end()
#define CheckBit(x, k) (x & (1LL << k))
#define SetBit(x, k) (x |= (1LL << k))
#define ClearBit(x, k) (x &= ~(1LL << k))
#define LSB(mask) __builtin_ctzll(mask)
#define MSB(mask) 63 - __builtin_clzll(mask)

template <typename T, typename U>
T ceil(T x, U y) { return (x > 0 ? (x + y - 1) / y : x / y); }
template <typename T, typename U>
T floor(T x, U y) { return (x > 0 ? x / y : (x - y + 1) / y); }

// typedef  __int128 lll;
typedef struct rafi
{
    ll type;
    ll idx1;
    ll idx2;
} R;

typedef struct sajeeb
{
    ll val1;
    ll val2;
} S;

typedef struct ohi
{
    ll l;
    ll r;
    ll u;
    ll d;
    int val;
} O;

const int N = 2e5 + 7, M = 1073741824, N1 = 64;

bool isPerfectSquare(long long x)
{
    // Find floating point value of
    // square root of x.
    if (x >= 0)
    {

        long long sr = sqrt(x);

        // if product of square root
        // is equal, then
        // return T/F
        return (sr * sr == x);
    }
    // else return false if n<0
    return false;
}

void sF()
{
    ll n, d, a, b;
    cin >> n >> d >> a >> b;

    if (n < d)
        cout << n * a << nl;
    else
        cout << (d * a) + ((n - d) * b) << nl;
}

int main()
{
    MNTDH_18
    // pow2();

    ll tc = 1, x = 1;
    cin >> tc;

    while (tc--)
    {
        // cout << "Case " << x++ << ": ";
        sF();
    }

    return 0;
}