#include<bits/stdc++.h>
#define ll long long
#define endl '\n'
using namespace std;
int main()
{
    ios_base:: sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    ll t;
    cin>>t;
    while(t--){
        ll n,d;
        cin>>n>>d;
        ll a,b;
        cin>>a>>b;
        ll sub=n-d;
        ll x=d*a;
        ll y=sub*b;
        cout<<x+y<<endl;

    }


    return 0;
}