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

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