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

typedef long long ll;
#define bug(a) cout << #a << " : " << a << endl;

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1; cin >> t;
    while (t--) {
        ll n,d;cin>>n>>d;
        ll a,b;cin>>a>>b;
        if(d>=n){
            cout<<n*a<<endl;
            continue;
        }
        cout<<(d)*a+(n-d)*b<<endl;
    }
    return 0;
}