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



int main()
{
    int t;
    cin >> t;
    while(t--){
        long long a,b,n,d,ans;
        cin >> n>>d>>a>>b;
        ans=d*a+(n-d)*b;
        cout << static_cast<long long>(ans)<< endl;
        
    }
}