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

void solve() {
    int n,d;
    int a,b; 
    cin>>n>>d>>a>>b;

    int total_cost;
    if(d>=n){
        total_cost = n*a;
    }else{
        total_cost = d*a + (n-d)*b;
    }

    cout<<total_cost<<endl;
}

int main(){
    int t;
    cin>>t;
    while(t--){
        solve();
    }
return 0;
}