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

    if(n<=d){
        cout<<a*n<<endl;
    }
    else{
        ll x=n-d;
        ll cost=x*b+d*a;
        cout<<cost<<endl;
    }
}

int main()
{
    int n;
    cin>>n;

    while(n--){
        solve();
    }
}