//on the name of Allah:)
#include<bits/stdc++.h>
#define int         long long
#define endl        "\n"
#define pi          2 * acos(0.0)
#define mod         1000000007
#define Mul(a,b)    (a%mod * b%mod)%mod
#define Add(a,b)    (a%mod + b%mod)%mod
#define all(x)      (x).begin(),(x).end()
#define allr(x)     (x).rbegin(),(x).rend()
#define gcd(x, y)   (__gcd(x, y))
#define lcm(x, y)   ((x/gcd(x, y))*y)
#define faster      cin.tie(NULL), cout.tie(NULL);
#define TC          int t ; cin>>t ; while (t--)
const int N = 1e6 + 7;
using namespace std;
void s()
{
    int n,d,a,b;
    cin >> n >> d;
    cin >> a >> b;
    if(n<d) {
        cout << a*n << endl;
        return;
    }
    cout << (d*a) + (max(0LL,n-d)*b) << endl;
}
int32_t main()
{   ios::sync_with_stdio(false);

    TC
    s();


}