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

#define st first
#define nd second
#define pb push_back
using ll = long long;
using db = double;
using ldb = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

void solve(){
    ll yp, lp, ys, ls;
    cin >> yp >> lp >> ys >> ls;
    if(ys <= ls){
        cout << ys * yp << "\n";
        return;
    }
    ll best = ys * yp;
    if(yp <= lp){
        best = min(best, ls * lp);
    }
    else{
        // winda plycej
        ll total = yp + (yp - lp) * 2;
        best = min(best, total * ls);
        best = min(best, ys * (yp - lp) + lp * ls);
    }
    cout << best << "\n";
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int t; cin >> t;
    while(t--) solve();
    return 0;
}