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

void solve()
{
    int n,d,a,b;
    cin>>n>>d;
    cin>>a>>b;
    int total=0;
    if(n<=d){
        total=n*a;
    }
    else{
        total=(d*a)+(n-d)*b;
    }
    cout<<total<<endl;
}

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