#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define MOD 1000000007
#define pb push_back
#define vi vector<int>
#define vl vector<ll>
#define F first
#define S second
#define yes cout<<"YES"<<'\n';
#define no cout<<"NO"<<'\n';
#define endl "\n"
#define all(v) v.begin(),v.end()
#define print(v) for(auto it:v) cout<<it<<" "; cout<<endl;
#define srt(v) sort(v.begin(),v.end());
#define rsrt(v) sort(v.rbegin(),v.rend());
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*b)/gcd(a,b)
#define PI 2*acos(0.0)
#define pr pair<int,int>
#define Tasrik ios_base::sync_with_stdio(0);  cin.tie(0); cout.tie(0);

int main ()
{
    Tasrik;
    ll t=1,n,a,b,d;
    cin>>t;
    while(t--)
    {
        cin>>n>>d>>a>>b;

        ll ans=0;
        if(n>d)
        {
            ans=d*a;
            ll x=(n-d);
            ans+=(b*x);
        }

        else
            ans=a*n;

        cout<<ans<<endl;

    }

}