/* 
*   @author:- SYEDA SADIATUL JANNAT TUSHI
*   date & time :2024-09-05 20:55:29
*   BANGLADESH , SYLHET. 
*/

#include <bits/stdc++.h>
using namespace std;

#define endl '\n'
#define rep(i, a, b) for(int i = (a); i < (b); i++)
#define ll long long
#define vi vector<ll>
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);

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

    ll ek=(d*a);
    ll dui=(n-d)*b;
    ll ans=ek+dui;

    if(n<=d){
        cout<<n*a<<endl;
    }
    else{
        cout<<ans<<endl;
    }
}

int main() {
    fast;
    ll T;
    cin >> T;
    while (T--) {
        solve();
    }
    return 0;
}