#include<bits/stdc++.h>
using namespace std;
int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        long long n,d,a,b;
        cin>>n>>d;
        cin>>a>>b;
        if(d<n)
        {
           cout<<(d*a)+((n-d)*b)<<endl;
        }
        else
        {
            cout<<(n*a)<<endl;
        }
    }
}