#include <iostream>
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,n,d;
    cin>>n>>d;
    cin>>a>>b;
    ll ans=0;
    if(n<=d){
        ans=n*a;
    }
    else ans=d*a+(n-d)*b;
    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;
}