#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
typedef long long int ll;


using namespace std;
using namespace __gnu_pbds;
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define mod 1000000007

ll lcm(ll a,ll b)
{
    return (a*b)/__gcd(a,b);
}


ll binExp(ll a,ll b)
{
    if(b==0)
        return 1;

    ll res=binExp(a,b/2);

    if(b&1)
    {
        return (a*res*res)%mod;

    }
    else
    {
        return (res*res)%mod;
    }
}


bool isPrime(ll n)
{
    for(ll i=2; i*i<=n; i++)
    {
        if(n%i==0)
        {
            return false;
        }
    }

    return true;
}




void solve()
{
   ll n,d;
   cin>>n>>d;

   ll a,b;
   cin>>a>>b;

   if(n<=d)
   {
       cout<<n*a<<endl;
   }
   else
   {
       ll ans=0;
       ans+=(d*a);
       n-=d;
       ans+=(n*b);

       cout<<ans<<endl;
   }


}















int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);





    ll t;

    cin>>t;
  //  t=1;






    ll caseNo=1;



    while(t--)
    {
        //  cout<<"Case "<<caseNo<<": ";
        // caseNo++;

        solve();


    }




    return 0;

}