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

void solve()
{
    ll n, d;
    cin >> n >> d;

    ll a, b;	cin >> a >> b;

    d = min(n, d);

    
    ll ans = (a * d);
    ans += ((n - d) * b);
    cout << ans << '\n';
}


int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int t = 1, tc = 0;
    cin >> t;

    while(t--){
    	//cout << "Case " << ++tc << ": ";
    	solve();
    } 
}