#include <iostream>
#include<algorithm>
using namespace std;

#define ll long long int
#define ld long double
#define endl '\n'
#define MIR183_Mac ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define yes cout << "YES" << endl
#define no cout << "NO" << endl

void solve() {
    // Your code here
    ll a,b;
    ll n,d;
    cin>>n>>d;
    cin>>a>>b;
    ll mx=max(a,b);
    ll mn=min(a,b);
    ll ans=d*mx;
    n-=d;
    if(n>0){
        ans+=n*mn;
    }
    cout<<ans<<endl;
}

signed main() {
    MIR183_Mac;
    // Always use ll, int may give TLE
    ll tt;
    // tt=1;
    cin >> tt;
    while (tt--) {
        solve();
    }
    return 0;
}