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

#define ll long long

int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        long long int n,d;
        cin>>n>>d;
        long long int  a,b;
        cin>>a>>b;
        if(n<d)
        {
            long long ans=n*a;
            cout<<ans<<"\n";
        }
        else
        {
            long long int  diff=n-d;
            long long int  ans=(d*a)+(diff*b);
            cout<<ans<<"\n";
        }
    }
    return 0;
}