#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;

	ll ans = d * a + (n - d)*b;

	cout << ans << '\n';
}
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int t = 1;
	cin >> t;
	while(t--){
		solve();
	}
	return 0;
}