#include<bits/stdc++.h>
using namespace std;
#define ll long long int 
int main()
{
    ll tc;
    cin>>tc;
    while(tc--)
    {
        ll n,d;
        ll a,b;
        cin>>n>>d;
        cin>>a>>b;
        ll x=min(n,d)*a+max(0ll,n-d)*b;
        cout<<x<<endl;
    }
}