#include <iostream>
using namespace std;
#define ll long long int
#define ld long double
#define endl '\n'
#define MIR183_Mac ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define yes cout << "YES" << endl
#define no cout << "NO" << endl
void solve() {
// Your code here
ll n,d;
cin>>n>>d;
ll a,b;
cin>>a>>b;
ll ans=0;
ans+=(d*a);
n-=d;
if(n<=0){
cout<<ans<<endl;
return;
}
ans+=n*b;
cout<<ans<<endl;
}
signed main() {
MIR183_Mac;
// Always use ll, int may give TLE
ll tt;
// tt=1;
cin >> tt;
while (tt--) {
solve();
}
return 0;
}