#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define all(n)  n.begin(),n.end()
#define sz(n) (ll) n.size()
#define endl '\n'
const double PI = 3.14159265358979323846;
const ll mod = 1e9 + 7;


void test_case() {
    ll n,d;
    cin >> n >> d;
    ll a,b;
    cin >> a >> b;
    ll ans = min(n,d)*a;
    n -= min(n,d);
    ans = ans + (n)*b;
    cout << ans << endl;
}

int32_t main() {
    //freopen("input.txt","r", stdin);
    //freopen("output.txt","w", stdout);
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    ll t = 1;
    cin >> t;
    while(t--) {
        test_case();
    }
}