#include<bits/stdc++.h>
using namespace std;
#define ll long long int
void solve(){
	ll n, d;
	cin >> n >> d;
	ll a, b;
	cin >> a >> b;

	ll totalCost = 0;
        
        if (n <= d) {
            
            totalCost = n * a;
        } else {
            
            totalCost = d * a + (n - d) * b;
        }
        
        cout << totalCost << endl;
}
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int t = 1;
	cin >> t;
	while(t--){
		solve();
	}
	return 0;
}