#include <iostream>

using namespace std;


int main()
{
    int t,n,d,a,b;
    cin>>t;
    for(int i=0;i<t;i++){
        cin>>n>>d;
        cin>>a>>b;
        int k= n-d;
        if(k<0){
            k=0;
        }
        int cost = (d*a) + (k*b);
        cout<<cost<<endl;
    }
    return 0;
}