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

using ll = long long;
using vi = vector<int>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vvi = vector<vi>;

#define f(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,a,b) for(int i = (a); i < (b); i++)
#define pb emplace_back
#define PB push_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
#define dump(x) cout << #x << "=" << x << endl;

#define int long long
signed main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	int n; cin >> n;
	f(_,0,n)
	{
		int a, b, c, d;
		cin >> a >> b >> c >> d;
		int ans = a * c;
		ans = min(ans, (abs(b - a) + a) * d);
		if (a > b)
		{
			ans = min(ans, (a - b) * c + b * d);
		}
		cout << ans << "\n";
	}



	return 0;
}