#include <bits/stdc++.h>
#define rep(i, a, n) for(ll i = a; i < (n); i++)
using namespace std;
using ll = long long;
using P = pair<int, int>;
const int INF = 1001001001;
const ll LINF = 1001002003004005006ll;
//const int mod = 1000000007;
//const int mod = 998244353;

void solve(){
  ll l, r, a, b;
  cin >> l >> r >> a >> b;
  ll ans = -INF;
  rep(i, l, r+1) {
    ans = max(ans, a*i + b);
  }
  cout << ans << endl;
  return;
}

int main()
{
  int t;
  cin >> t;
  while(t--) solve();
  return 0;
}