#include <iostream>
#define ll long long

using namespace std;

int main()
{
    int t;
    cin >> t;
    while(t--){
        ll sj, sl, rj, rl;
        cin >> sj >> sl >> rj >> rl;
        if(sj == 0) cout << 0 << endl;
        else if(sj <= sl) cout << min(sj*rj, sl*rl) << endl;
        else if(2*rl < rj) cout << rl*(2*(sj-sl) + sl) << endl;
        else if(rl < rj) cout << rj*(sj-sl) + rl*sl << endl;
        else cout << rj*sj << endl;
    }
    return 0;
}