#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 tk, tk2;
        cin>>tk2>>tk;
        if(n>=d){
            ll sub=n-d;
            ll x=d*tk2;
            ll y=sub*tk;
            cout<<x+y<<endl;
        }
        else{
            ll tm=n*tk2;
            cout<<tm<<endl;
        }

    }


    return 0;
}