#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define nl '\n'
int main() {
    int t; cin >> t;
    while(t--){
        ll n, k; cin >> n >> k;
        ll a, b; cin >> a >> b;
        ll ans = min(n,k)*a;
        ll baki = max(0ll, n-k);
        ans += baki * b;
        cout << ans << nl;
    }
}