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

#define ll long long
#define nl "\n"
#define ws " "
#define FASTIO ios_base::sync_with_stdio(false); cin.tie(NULL);
#define MOD 1000000007

typedef vector<int> vi;
typedef vector<pair<ll, ll>> vii;

int main () {
    FASTIO
    int t;
    cin>>t;
    while (t--) {
        ll n, d, a, b;
        cin>>n>>d>>a>>b;
        cout<<(min (n, d) * a) + (max (0LL, n - d) * b)<<nl;
    }
    return 0;
}